Spoiler alert: I'm picky. Deal with it. Couple things I don't tolerate:
-Most forms of slash/fem-slash. In the DP fandom, I'm okay with Pitch Pearl, but nothing more.
-Along with the slash thing, I don't do incest.
-Mary-Sues and Gary-Stus. I'll tell you if you have one.
-Generally anything non-canon when it comes to relationships
-In the DP fandom, Danny/Dani father/daughter. That's just... ew.
-On that note, I don't really care for any other father/daughter or father/son or whatever pairing that's non-canon
-Somewhat DannyxSam in the DP fandom. If you write the relationship well, I can handle it.
-Overused plots, unless there's a unique twist to it that's not too out-there.
-Also in the DP fandom: Taking Clockwork and Vlad out of character. I have definite expectations for the former, and I can tell when the latter is OOC (but I'm not good at getting him IC)
Things I WILL tolerate:
-Most AUs. It depends on a lot, though.
-Any character-bashing. It's fun to point out the negatives in a character.
-Bouncing theories back and forth, especially in the DP fandom. I need to work on mine some, and I love seeing what everybody else thinks.
-In the DP fandom, non-canon powers or abilities of Danny's (whether he gains them in the story or has already had them), just so long as they're believable
-Character death (for generally anybody)
-Mild violence (but, as noted, nothing M-rated)
-In the DP fandom, pretty much any kind of post-"Phantom Planet" fic.