I have a lot of sympathy with those who don't like to warn at all. There are times when giving a warning gives away the story, gives away one's punchline, and if it's the sort of story that needs that moment of impact, then what's the point of even writing the story at all--if you are then required to spoil it?
On the other hand, the audience for fan fiction differs from the audience for published novels. When one's readers will be looking for specific types of stories involving their favorite characters and worlds, it behooves the writer to aid that search in a way that would be wrong and foreign to a novelist.
I tend to try to get away, these days, with limited amounts of warnings. I categorize fics as gen or het (and list the main pairing for het fics), but I tend not to think of those as warnings per se. I was well-trained in my first on-line fandom--The Sentinel--to warn for character death, although such warnings have gotten more abstract. I do warn for sexual content (especially if it's in an abusive or violent context, not that I write much of that), and for violence if it's above what the source canon would include.
I've occasionally warned for OCs. :) And a couple of times I've used a non-specific "triggery" warning, without categorizing what that might mean.
Much more than this, I feel, would get overdone and silly. Which is fine if one is writing a humor fic, but not so much otherwise.
16 – Summaries – Do you like them or hate them? How do you come up with them, if you use them?
17 – Titles – Are they the bane of your existence, or the easiest part of the fic? Also, if you do chaptered fic, do you give each chapter a title, or not?
18 – Where do you get the most inspiration for your fics (aka "bunnies") from?
19 – When you have bunnies, do you sit down and start writing right away, or do you write down the idea for further use?
20 – Do you ever get bunnied from other people's stories or art in the same fandom?
21 – Sequels – Have you ever written a sequel to a fic you wrote, and if so, why, and if not, how do you feel about sequels?
22 – Have you ever participated in a fest or a Big Bang? If so, write about your favorite experience in relation to one. If not, are there any you've thought about doing? And if not, why not?
23 – When you post, where do you post to? Just your journal? Just an archive? Your own personal site?
24 – Betaing – How many betas do you like to use to make sure there aren't any major flaws in your fic? Do you have a Beta horror story or dream story?
25 – Music – Do you listen to music while you write? Do you make playlists to get into a certain "mood" to write your fic? Do you need noise in general? Or do you need it completely quiet?
26 – What is the oddest (or funnest) thing you've had to research for a fic?
27 – Where is your favorite place to write, and do you write by hand or on the computer?
28 – Have you ever collaborated with anyone else, whether writing together, or having an artist work on a piece about your fic?
29 – What is your current project or projects?
30 – Do you have a favorite fic you've written? What makes it your favorite? And don't forget to give us a link!