Depending on the situation I don't mind any content at all. It really depends on the flavor of the comic. For example, Weesh would be ruined by the introduction of anything beyond a strict G rating. At the complete other end of the spectrum is Jack. If it were made into a movie, Jack would NR (not rated) or X. It would leave NC-17 in the dust, not just for (pleasant) sexual content, but also murder, molestation, torture, gore, rape, pedophilia and religious themes (among others). Both of these are on my 'check daily for updates' list.
The other 'don't mind' content is visual. My tastes run the gamut from DMFA to Castlevania RPG to Antihero for Hire. You'll notice that there's some fair variation in the art there, including sprite comics. As long as the visuals back up the story but don't dominate I'm not going to complain. But once again, this can also be subjective to the story's 'flavor'. I don't think I'd read MegaTokyo if it were done in stick figures.
Ultimately there are only two things that drive me from a comic. First, overly long/unfinished story arcs. I dropped Sluggy Freelance for exactly this reason. Oh, and killing Zoe. I gave it a month after that to prove she wasn't Killed Off For Real and then dropped it.
Second, jumping outside the flavor of the comic. If you've got a light, kid friendly comic, don't go and start making it violent/provocative/political/etc. I read it because it's good the way it is. And that goes for violent/provocative/political/etc. comics too. Just don't go changing things that work or you're going to just piss off your fanbase.