There's that saying "don't judge a book by its cover".

But most of us like pretty things, so we do anyway.  Colour attracts the eye and beauty attracts the eye.  Too much of the first and your eyes hurt and too much of the second makes you see it as the norm.  I wouldn't say it makes you immune to it, but it becomes something you expect and therefore the standard by which you judge.  For example, if you go to a ball, you expect everybody there to look gorgeous and for those wearing dresses to be wearing beautiful dresses.  Somebody who turns up in jeans and a t-shirt jars with that, because, while they may look beautiful in another context, by the standards of the context you've set, they are below par.

If most of the girls in your school make themselves look gorgeous by wearing makeup, then those who don't wear make up stick out like a sore thumb by virtue of the fact that they haven't enhanced their looks.  It's the same with book covers.  If all the books on the shelf have beautiful photo-like covers, the ones that don't have gorgeous artwork on the covers are going to stand out.  If all the stories you're reading on line have amazing posters, then those that don't are going to stand out.  Sometimes standing out is a good thing.  Sometimes it's not.  Moving away from talking about schoolgirls wearing makeup - because whether you do or you don't doesn't make you any less of a person - when it comes to stories, if you're accustomed to seeing them with amazing covers, those that don't have them will feel like they're lacking in some way.  If they have amazing artwork, however, then your eye is going to be visually arrested.

Is it worth making your work visually appealing?

Heck yes.

TL;DR

Avoid: Lots of white space; lots of clutter.

Also avoid: A poster that doesn't suit the vibe of your story and a background that doesn't go with either the cover or main poster (it's better not to have them than to have bad ones); backgrounds with too much going on; making your text look so pretty it distracts from the actual story; weird formatting (that distracts from the actual story); text alignment that doesn't start on the left (if you're writing in English); unfriendly fonts; colours that aren't dark for text; long blocks of text; bold, underline, strikethrough (and other formatting); overusing italics, especially where the natural stress doesn't require italics to fall where it does in the sentence.

Go for: Posters and backgrounds to add colour (make sure that they fit the vibe of the story); default formatting unless you know what you're doing in terms of the science and psychology of reading ((default) near-black, sans serif font of a similar kind to this, size 12, aligned left, white space between paragraphs, no paragraphs longer than about 500 words, minimal italics, (default) just-off-white background).

VL;RA/Reasons for the Above

Even though the revamped version of the site functions much more smoothly than the old and has more features, I stuck with the older version for a while because it was more visually appealing.  It had much more colour; the colours were more vibrant and contrasted more; and even though the amount of white space was roughly similar, it wasn't as invasive as I find it on the new site.  The new site I guess looks more professional because it all operates on colours of a much more similar scheme: most things are blue, and then you get a bit of green, a lot of grey, and a smattering red.  But with the exception of the green and the very, very occasional reds that are basically orange (basically your name), all the colours are in the softer range of the palette, either by being dark or by being light.

The old site, thought, had much more of the bright green, notifications in red and orange, bits of yellow, and some block black at the top that made the white pop.

I personally think that the colour scheme of the new site is much more cohesive, but it leaves too much noticeable white, and too much noticeable white is bad.  This is why adding other colours is a Good Thing.

It was actually the new site that made me think seriously about a DP on my profile page.  The old site had enough colour going on for me not to be too bothered about seeing a grey person-shaped-thing on a space of white instead of a picture or gif if I went onto somebody's profile.  The new site, though, backed that onto pastel blue, so you have pale grey on pale blue on white, which really, really looks like something is missing there.

And so of course I just happened to choose the one gif of Xiumin that serendipitously fit more or less with the colour scheme.  Oh well.

The green version of the site is more vibrant than the blue, so I sometimes use that, but I feel like there isn't enough green.  It appears in, like, one area (up at the top) and everything else is still in blue, so it kind of looks a little out of place.  But it still makes you notice the white less, which I guess is a good thing.  The black and white theme just removes colour from the blue theme.

Anyway, this isn't to discuss the colour themes of AFF, though I still think it's an interesting topic.  The issue at hand is that the majority of site users (it appears) use the blue theme, and the blue theme leaves a lot of white space.  There are three areas you can minimise that invasive white space.

1) your profile.

2) your story.

3) the snapshot of your story that appears on the front page and under the tags listings.

The profile's an easy one to sort out by just grabbing a picture or gif to use as your DP.  Totally up to you whether or not you do that.  That's not a discussion that needs to happen here.

The story... like I said, most people judge a book by its cover, even though everybody knows they shouldn't.  You can lead a horse to water, but you can't force it to drink and all that.  It is a good idea to at least have either a background or a poster for your story.  You don't necessarily need a poster at the start of every chapter, but on the foreword/description page, I'd highly recommend it.

The poster is the place to let loose creatively, or the place to let somebody else let loose creatively.  If you don't fancy photoshopping yourself, then there are some absolutely great photo manipulators on the site who make absolutely fabuous posters, so it's worth paying the karma for that, or asking a friend nicely.  Whether you want something simple or elaborate is up to you (or the poster-maker, at the very least).  You want something eye-catching.  That means something that's not too bare, nor too cluttered (yes, I'm aware that telling you the porridge shouldn't be too hot or too cold but just right is unhelpful, but striking that golden balance is going to depend on the story and poster in question).  The poster should reflect the vibe of your story, too.  A mystery, thriller or horror story is probably going to be best suited with dark colours (or more darkness than light colours, definitely, though a few bright colours like red probably won't go that amiss), and won't need very much on the cover to get the vibe. A humour or fluff story, on the other hand, will have its vibe best brought out by bright and light colours.  Humour and action can both comfortably be suited with either having not very much or having quite a lot on the cover.  In fact, the more light-hearted it is, the more on the cover you can probably get away with.

Compare the five covers below:

Even without looking at the titles, you can tell an awful lot from the covers, particularly from how "busy" they are and the colour schemes that they use.  Going with the one on the right first (which is the cover for this guide - I probably could have used a colour other than blue, but oh well), the bland simplicity of it basically tells you "read at risk of boredom!" (or, alternatively, "non-fiction here!" because there's nothing going on on the cover other than writing).  It tells you that it's pretty serious.

Going from right to left - or, from least busy to most busy - next up is the cover for TBBC.  The colour scheme is black and red (and white).  The white tells you it's not a totally tragic story, but the black and red definitely give it a sombre vibe and suggest something vaguely actiony/involving danger that's potentially life-threatening.

For Rogue, in the centre, the colour scheme (half black/grey) again lends it a rather sombre vibe.  The yellow stands out more as a result.  I won't be cheesy enough to say that the yellow's a note of hope or anything, but it does lessen the seriousness from being quite so intense.  The picture is also "busier" than the previous one - visually, there's more on it.  TBBC has two heads, the title, a gun, and then colour.  Rogue has the dark colour scheme, a bloody hand, two people and weapons.  Because there's more there, it gives more of a feeling of stuff going on.  That's not to say that nothing ever happens in TBBC because it has less on the cover, but rather that the cover of Rogue suggests more action to the reader, which is good, because it's supposed to be an action story.

On Ebb is busier still.  It's probably the most colourful of the five, but the colours are darker than those for DLWL.  The font also stands out the most of all the fonts.  The way that things are set out - floating symbols, a character holding a crystal - along with the font suggest something vaguely in the fantasy genre.  The tree being struck like lightning suggests pending disaster.  The stuff going on - the tree, the floating symbols, the person - all taking up prominent space towards the front rather than one dominating over the other suggests a packed plot.  (I told myself I wasn't going to actually analyse the imagery.  Failed that one.)

And then, DLWL.  Definitely the busiest.  It's got layers - photos on top of photos - with the most important one in front.  But the colour scheme is a pretty light one - white, light blue, pink.  That all suggests light-heartedness and fluff.  Not humour, per se, but most stories that are heavy on humour also have a lot of light-heartedness and fluff.

For somebody who clicks on your story, seeing the cover before the description automatically puts them into a certain mindset before they read the description.  It can either undermine or enhance your description and title, and as we all know, the description and title are vitally important for setting the right vibe.

I mean, if you saw this:

And then read this:

Semi's graduation present was a death threat from the country's biggest mafia boss and a hasty eviction to the United States.  Nine months later, she's in hospital making a near miraculous recovery from an assassination attempt from said boss, known merely as Luhan, who has been persistent enough to track her all the way across the world -- to prevent her from using the evidence she has on her phone, from when she filmed a murder...

You're going to have a lot of trouble marrying the two up.  They just don't fit.  Quite obviously so.  Everything about the poster screams light-hearted fluff and funsies and laughter for all! but there is nothing of that reflected in the description, and it really jars with the title.

(Disclaimer: I do actually love that poster.  It makes me laugh so hard every time I see it.  One of my TBBC readers made it as a joke.)

On the other hand, if you see this:

With that same description, they clearly fit together a lot, lot better.  Asassinations?  There's blood.  The colours suit the mood (they may be largely pale, but they aren't "bright" pale colours (if that makes sense) like blue or orange).

The first poster clearly undermines everything in the prologue.  The second poster enhances it.  It's like the difference between putting makeup on badly and makeup on well.  If it's that bad, you're probably better off without it.  You're better off not having a poster than one that doesn't suit, to be quite frank, when it comes to your cover.

The other thing to remember is that you can put your cover as a thumbnail.  The overall colour (since a thumbnail is small) will be what attracts the attention of somebody as they scroll down the front page or through a page of tags.  Since there's so much white space, even if everybody has covers, the thumbnail breaks that space and attracts the eye.  But the thing is, not everybody does have covers (or they don't use the thumbnail setting if they do), which means that if you do have a cover and put it on the thumbnail setting, you stand out even more.  You're that book with gold patterns on the spine in the bookshop while everybody else's is navy blue or black.

Covers are important.  People judge books by covers.

Once a person has actually clicked on your story, though, whether or not you have a poster every chapter, there is still a heck of a lot of blank white space.  It's not that visible, admittedly, for people reading from their phones, and therefore not as jarring (basically, as I see it, the old AFF layout and colour scheme was set up for computers, and the new one is much more mobile friendly), but if you're on a computer, then the white space is everywhere.

The easiest way to get rid of that white space is to have a background.

Unlike the cover, though, you don't want the background to attract too much attention.