If someone is an exceptional player and really stands out, I'll give them the commendation. If everyone was pretty much equal, I just give it to the dps since they seem to be the hardest to get commendations as. People who beg for them will never get one from me, no matter how well they played.

I know it will never catch on, but it would be cool if tanks/healers gave each other commendations, and if dps did the same. Everyone would leave the dungeon with one, baring those who clearly don't deserve it.