I'm going to bring up something that a lot of people seem to be up in arms about. That is the idea of character's having power beyond what they can project using game mechanics.
Personally, I feel this issue actually has two questions bundled up in one.
First, there is writing your character non-game-engine power over your own NPCs. In my opinion, this is generally not particularly problematic as it is something that other players can ignore if it is something they do not enjoy or does not fit into their view of the gameworld. It is a big cluster and there is room for everybody if you play nice.
The second case is writing your character power over the gameworld and/or other players' NPCs and characters. Which in my opinion needs OOC consideration and courtesy to work well. It
can work and lead to fun RP. But it can also lead to trouble with character or world consistency, feelings of godmoding or other OOC friction.
I would personally be quite careful about the second kind when I do not know the other players. In most RP subcommunities there seems to be an accepted level of character power that is considered "appropriate", and going beyond that without previous OOC negotiation and agreeing on said power use with other players is felt to be bad form. Sometimes that line can be in unexpected places.