Personally I pick my player commendations by people who do their jobs well, and are party players. I mostly commendate the tank since theyre the ones usually pulling the agros off me as a healer allowing me to either holy or cure people without constantly having to keep myself alive though I don't mind doing so, its hard to keep everyone alive if your healer is being attacked by 6 monsters at a time. If a tank does this well I tend to commendate them by that. Then there are DPS who can do the same thing and play very well. If I see a DPS doing such I'll end up commendating them instead.
I also don't like it when people don't give new people the chance to view the cutscenes in the dungeons and get mad at people if they mess up for the first time. It's a game and mistakes allow people to get better skill at playing the game, we should be encouraging not negative towards new players.