I always give commendations based on the same criteria I use for my own self-betterment.

Did the player attempt to perform his/her job?
Did the player go above and beyond the call of duty? (healer did damage, DPS buffing/debugging, tank dodged AoEs)
Did the player communicate?
Was the player pleasant?
Did the player explain mechanics and answer questions?
Did the player wait for someone who fell behind, or for someone who was new?
Did the player help newer members achieve needed goals, experience, or loot, even at the cost of their own time?

Obviously not all of these are applicable in every situation, but for the most part they work well.