Things everyone in fandom "warns" for:
-Pairings
-episode spoilers
-AUs
-Het, slash, gen (often by including pairings)
-Character(s)-centric versus ensemble (sometimes through the summary)
-Whether you've gotten your fic beta'd (authors notes)
-Crossovers
-Plot (sometimes through the summary)
-Porn
-Type of porn
-Length of porn
-Kinks in that porn
-Number of people involved in that porn
-Whether it's only porn, or whether you've tried to fit some plot in there
-Whether your fic will be shitty badfic (author name)
-OOC
-Characters turning into animals/lampshades/etc.
Things that would apparently be too spoilery to warn for:
-Non-con
-Dub-con
-Abuse
-Etc.