Your potential reader is a mouse, your story is the mouse trap, and the description is a big fat piece of cheese.

 +  +  = 

Cheese as in, something that looks genuinely worth venturing into the trap for, not something cheesy that makes you cringe.  And if you go into writing the description for your fanfic with that mentality, it's going to be pretty hard for you to go wrong.

If you misfire with your title, you have three sentences to convince the browsing reader to click on your story.  Provided you haven't coded in a lot of formatting, in which case you have none.  Make those three sentences count.

TL;DR

Avoid: anything that does not relate directly to the plot.  Do not introduce yourself.  Do not pass Go.  Do not collect $200.

Also avoid: spelling and grammar errors; telling your readers that "x is just an ordinary teenager" (they want to know why they should read about your character rather than anybody else's - "hi, I'm ordinary" is unlikely to cut it); giving away the entire plot (especially the - which can be done if you make it too obvious as well as through outright stating it); trying too hard; directing questions to the readers (which they will answer before simply moving on to another story); sycophantic quotes; anything generic enough to be swapped with a different title/characters without people realising they're from two different stories.

Go for: a gripping, punchy first sentence; protagonist, goals, conflict and stakes; the actual story (without spoilers, of course); something that captures the vibe and tone of your story (e.g. a story in the humour tag ought to have a light-hearted or humourous tone).

VL;RA/Reasons for the above

For description, read blurb.

You may have noticed that only the first couple of hundred characters of your description get nabbed when it's displayed on the front page.  Your first sentence, therefore, is your most important.  Somebody who skips over your title may get reeled in by your description, so it really needs to pack a punch.  There's no point repeating titles, tags/genres, putting your name or anything like that in the description, particularly at the beginning, because it loses you advertising space for your story.  (Not to mention they're already there anyway.  There's no need to repeat.)

Also, if you're thinking about starting your description with "hi, guys!  I'm xxx and I'm back with my new fanfiction"— forget it.  In the nicest possible way, just forget it.  I want my cheese.  If all you're going to show me is the trap, I'm going to be heading in the other direction as fast as possible.  In not using the description to sell your story from the very first letter, you alienate the entirety of your target audience except the ones who already know you and who have already read your stuff, and they probably already know about your story, so if they're not reading it already, they probably won't.  This is shooting yourself in the foot, marketing style... okay, it's shooting yourself in both feet, marketing style, absolutely spectacularly.  If you want people to read your story, you have to pitch correctly to the target audience.  You don't do that by not actually pitching to them.

I mean, that's a rule of thumb about getting anybody to do anything for you, ever. You have to pitch it the right way, at the right time, to the right person.  If you aren't doing all three of those things, then you already have yourself a problem before you've even started.

If you start with anything other than launching straight into an engaging sentence for your description, you are:

1) alienating all the people the tags (and title) would otherwise draw in - people search by tags.  If somebody is in the action tag and sees a description that starts off with Lee Jungmi had grown up hot wiring cars, they're probably going to pick that story over Guess who's back, everybody?  Me!  And this time, I'm writing an action story.

2) silently screaming that you don't think your story can stand on its merits alone - if you have to introduce yourself first and tell people you're a nice person, you're marketing yourself, not your story.  You can be the nicest person alive, or an absolute spoilt brat, but the people surfing through stories are looking for stories, not friends (or brats).

3) not actually giving people a taste for what your story is like.  If you don't know anything at all about a story, it's kind of hard to want to read it.

Sure, you'll still get some people reading it whatever you do, but there are hundreds of thousands of readers on this site, and there are also hundreds of thousands of stories, and that means that a lot of people are vying for attention when it comes to their stuff being read.  If you want a large catchment of readers, the best way to get it easily is to stand out.

The blurb, or description, is basically there so that people under the genre (or tag) who have stopped by in that section and looked further than the title can go "oh, hey, this story might be for me" in the space of a couple of seconds.  If you have to click on an extra link (and if you're using data (and that therefore costs money) or, say, in class or somewhere else you shouldn't be on your phone, the last thing you want to do is be clicking extra links – you want to be able to scroll through as soon as possible), it's an irritation.

So, as the author, it's your responsibility to get somebody to click on that hyperlink to take them to your story, rather than overlooking it as they scroll down through the other twenty or so on the page for something that jumps out at them more.  Save all the personal stuff for the foreword, where it's not going to prevent anybody from clicking on your story.

Unless you're using lots of formatting (yes, it looks pretty, but I don't recommend it if you want your description to actually show up), you have about 300-400 characters to make your case for why somebody needs to read your story, and why they need to read it right now.  Because that's exactly what your description should make them feel.  Those 300-400 characters translate to about three sentences.  It's just over the length of the description if you want to advertise something.  Anybody who's put up a bid for an advertisement will know that that's not a lot.  But the thing about a description is that, unlikely the advert, you have more available to you than just three sentences, which means the stress of packing it all into such a small space isn't there.  It might as well be, though, because everything still rests on that first sentence.  But if the first sentence packs a punch, people are likely to follow through and click.

How do you write a first sentence that packs a punch, then?  Write a statement about the instigating incident of the plot.  That's all it takes. What is it that kicks the plot off?  (Also, if this doesn't happen within the first five chapters of your story, preferably before, you're doing it wrong.)  Don't write a question.  Don't use a quote.  Don't make it long.

(And yes, I realise that TBBC's description starts off with a quote and a question.  Sometimes, you can break the rules.  You just need to make sure that you're doing it to greater effect than sticking to the rules would.  If you're going to ask a generic question, like "what if you found yourself engaged to somebody you don't know?" or quote "love conquers all", there is literally nothing that makes your story stand out.  And not making your story stand out is a big, big problem, because it means you won't attract as many readers as you should.  I guess TBBC worked because it showed that the situation was unique.  I mean, if I'd started it "would you film a gang lord beating somebody to death in a dark alley?  Because Oh Semi did", I can guarantee that it wouldn't have half the number of readers it does.)

When I first put up Rogue, one of the people who commented on it said that the description had drawn her in and that she'd subscribed immediately, having seen it under the tags list, without even being aware that I was the author.  She was a TBBC reader.  That's how powerful a description can be.

This is the first sentence of Rogue's description:

Chaos erupts in Cho Miran's department for the Secret Service on her second day there when notorious enemy spymaster Oh Sehun turns himself in at their local headquarters.

In this one sentence, we have:

1) instigating incident: enemy spy master turns himself in: chaos ensues

2) introduction to the main character, Cho Miran

3) hint of immediate conflict that needs resolution - there's chaos

and 4) the question why the hell did a notorious enemy spymaster turn himself in?

The sentence is probably far too wordy - choas erupts on Cho Miran's second day at the Secret Service when notorious enemy spymaster Oh Sehun turns himself in would probably work just as well, if not better.  You can go overboard as well as underboard.  Also, there's the tone.  The tone of the story quite clearly suggests it's serious, and that there will be intrigue.

Compare that to the first sentence from the description for Deer Luhan, With Love:

A case of dyed pink hair, an argument with la madre, and a freak encounter with an EXO member in the park after being hit in the head by a plate find Leigh Lee on a plane to Seoul the next day, with little recollection of how she got there and EXO's maknae intent on calling her hyung.

Here, we have:

1) inciting incident: a case of mistaken identity

2) introduction to the main character, Leigh Lee - who has bright pink hair, argues with her mother, looks like an EXO member, is mistaken for a boy, and has temporary memory loss

3) the tone is overall humorous: there's a case of mistaken identity and the character is presented as quirky

4) conflict: how is this situation going to be resolved?  And also, what's going to happen when people realise she's not a boy, let alone who they think she is?

Again, it's probably too wordy, but it does what it needs to do, and it works.  The entire story is humorous - which is clearly reflected in the first sentence of the description (and if your story's tone and the tone of your description don't match, you're doing it wrong).  The essentials are all laid out.

And the last one - and my personal favourite - On Ebb.

Sehun thinks he's got his hands full when his best friend, the princess of Exoplanet, has her memories wiped for the seventh time in an attempt to curb her volatile magical powers.

Here, we have:

1) not quite inciting incident, but something clearly very important to the plot - the princess has volatile magical powers... and no memory

2) introduction to one of the main characters (the other is Chen)

3) the tone is pretty light-hearted, as is Sehun's side of the story with the princess

4) promise of further troubles and conflict: he thinks he's got his hands full with a best friend unable to remember anything with magical powers that could go out of control at any moment, which looks bad enough - but the fact that he thinks that means things are obviously going to get even more out of hand.  Especially since, if this is the seventh time the princess's memories have been wiped, it's obviously not a method that's working very well.  So on top of the other potential problems, what's going to happen when those memories come back?

The key here is mixing subtlety with pulling absolutely no punches.  You want it to be compact and engaging, but not to try so hard to engage the reader that you end up putting them off.  If you start with a question, e.g. "what would you do if...