I already posted some of mine in another thread.
But yes, godmoding is one of them. 'Godmoding' or all too easily defeating other characters (especially other players' characters) in a fight. Stealing other player characters' tools or weapons without consent from the character's player, especially if they have special significance and aren't easily replaceable or can't easily be stolen back. All this is stuff that should be avoided.
Another pet peeve of mine is when people disrupt the narration by putting an OOC remark in parenthes, such as in the following example:
He would have reacted sooner, but he was still weary from the last battle (OOC: and the guy who played him was stuck in traffic and didn't get home till late). Before he could stop them, the door had already closed and the Masters were already on their journey.
That is jarring, and throws us out of the story. If you have to have an OOC comment, keep it separate from the IC portions of the post. Here is a better way of doing it:
OOC: Sorry I'm late. I got stuck in traffic.
IC:
He would have reacted sooner, but he was still weary from the last battle. Before he could stop them, the door had already closed and the Masters were already on their journey.
There, that's much better. The OOC is much less intrusive and immersion-shattering.
Anyway, those are a few of my RP pet peeves.