Interesting questions :)

First of all, I think all writers love comments, and I'm no exception.
It sort of makes you feel like you're not alone with your story, but like somebody is experiencing it with you.
Writing fanfics honestly sometimes seems a bit pointless. But if you get comments at least you're not alone.
Sort of like the difference between skipping school alone, or with your friends.

The only comments I would rather be without, is when people tell me what to do, like to write different pairings, different bands etc. I don't mind the update soon comments when they are polite, but it's a lot nicer and more encouraging when people say "I'm looking forward to your next chapter" or something similar.. which sort of means the same, but is less pressuring.
And I would hate to have people trashing the idols I write about (like the real ones... it's different when they criticize the characters in my stories... because sometimes they deserve it lol), but luckily I have never gotten such a comment.

My absolutely favorite types of comments are when my readers are flailing, or any type of being invested in the story. Like comments about characters or events, directed to them or outside, either way is fun. Or if the readers are just flailing because the idol is cute/hot/awsome, even though it has nothing to do with my writing or my story, just the idol himself, I like it lol.

Most of the time I comment on the stories I read, and I try to answer comments semi-regularly (depends a bit on the story though).
Whether I do or don't has more to do with myself than anything else. Because sometimes I'm in hermit mode, and can't really talk to people so easily. Also, I tend to sound more harsh when I'm tired, and I don't want to be mean to anyone, or cause misunderstandings. So then I usually don't comment/answer comments.
So yeah, depends on my mood more than anything else.
But I'm always grateful to get comments, even when I don't answer them.

Commenting get scary at times. I never want to offend the writer. Sometimes i have thoughts on things or opinions. I sometimes want to share but i'm afraid the author would think I'm critiquing them.
I like comments on my story and would't mind any kind but i get so scared. Thats my personality tho.
Sometime when i comment on stories I'm more talking to the characters.
I get really into stories that i feel like I'm probably TOO into them and i stop myself from writing a long comment and make myself just writer a 'great story.'

what usually makes me comment is if there's a crazy cliffhanger that leaves me on edge wanting more- HAHAH. i usually just cry or moan about the cliffy in the comment section.

or the story has ended and i want to thank the author for writing the story ; v ;

Do you want comments and what kind of comments? (positive/negative critique, honest opinions, support or 'gushing' and so on) Yesssss, any kind of comment, all comments. I want to read what my readers think, be it positive, negative, honest, critique, support, gushing. I'd even go so far as to say that I'd like hate as well, because if someone bothers to leave hate they must really think that my stories either or that I'm more "successful" than I deserve. Anyway - I usually go so far as to beg for comments because I'm pathetic like that.
What makes you comment or not comment? Every time I read a story I tries to comment. I don't normally comment on every chapter, but I'll comment when I reach the end of a story, on-going or not. Because I know how much comments mean to me so I assume they mean just as to other authors.
When you ask for comments, what kind of comments do you want? Like I said - any comment, LOL. Bring it to me, anything, everything.
Do you answer your readers comments? (why/why not) Yes, I do. Because usually people take the two seconds to write a comment so I can take two seconds to answer. Sometimes it's a simple "thank you", other times my readers leave questions that of course should be answered. If they leave questions that could be spoilers I'll answer with something witty (or try to be). Because I want conversations and this is how I get it. I get inspired by my readers as well.
OH and also, do you prefer comments on the plot and characters and/or comments kind of directed to the characters? Any comment, haha. I appreciate those who talk about the story, but I've never had comments directed to the characters. I would probably not be able to answer those, so. But maybe we'll see? Idk. ^^

I love comments - is it visible? XD

As author ofc I really appreciate comments. Just like wooswitchofsuburbia said.. I like when they left their opinion and any kind of comment, but I don't like when they only say "update soon authorssi"
As reader I comment when I enjoy reading it.. The flow just good not too fast or too slow, and if I get the emotion from reading it I will absolutely left comment.
Yes I reply their comment. But I don't reply " update soon" comment.

Honestly, I desire comments more than anything. They're what drives me to keep writing because I feel like real, actual people are looking at my work and appreciate it. I don't really need comments that just say: Update soon! I prefer that they say what they're enjoying about the fic (and I really love constructive criticism, too).

I always try to comment whenever I read anything. It's only fair, since I desire comments so bad. You get what you give, right?

And I almost always answer my commenters. I think if you can, you should. Let them know that you appreciate their opinion or input. It might also show that you're quite friendly and easy to talk to, and that their comments are also being heard. No one likes talking to a brick wall, we all want to be acknowledged.