I usually commend based on following:
- Priority order: Tank - Healer - DD. This will change if one of them perform better than others.
- Things that detract my commend judgement: Looting need over greed for low level gears, which usually leading me to need gears I can get. That person still has chance getting my commend though. Gets worse if I greed on earlier loot that I can actually need, then others picking need, usually eliminating that person from my commend list. lol