I'm usually quite comfortable writing up to PG-13 for most things; I've gone easily up to R for language and violence. If the story called for it, I could imagine going up to NC-17 for violence (but yikes, it would have to really need it, because that would be tough for me to write).
I've written a little bit of R/NC-17 sexytimes fic, but I don't know that I would ever be comfortable posting it for everyone else to see. :) Usually kissing or some nakedness fading to black is as much as I am comfortable with, there.
But honestly, I don't tend to think much about ratings until I go to post. Then I figure it out. The story determines what the content it; figuring out how to present that content is the last things I do.
15 – Warnings – What do you feel it most important to warn for, and what's the strangest thing you've warned for in a fic?
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!