For me, I usually look for:

-Good attitude
-Works well with the group
-Knows how to play their class/job
-If needing help, will listen to advice without being apprehensive
-If giving help, doesn't come off as snobby or rude

Pretty much, so long as they aren't an ass to the other party members, I'll consider anyone for a commendation. After that, it's whoever had the best attitude throughout.