Most things I report are: spam, advertising, threads in wrong section, posts not following 4/25 rule, off-topic posts, revival of dead threads, and also if they are disobeying the rules of certain threads.
GE's What Are You Listening To? thread has rules that comments need to be included when posting the song, and also no embedding of YouTube videos allowed. I tend to report loads of posts in that thread for those two rules alone. And also in TC's Quick Trade Thread I tend to report any post that isn't mentioning which game or generation they are trading from, since a rule there is to specify the generation/game you are using.
Yeah I tend to report things a lot, and in loads of sections too because mods can't catch every little thing easily.