i mostly run tank+heal so... because i have no other option ?^^
no seriously, under normal circumstances i'll more or less commend someone randomly, like i don't really gain anything from not doing it at all so whatever.
aside from that i go : helpfullness>general chattiness>just being really awesome at your job>not sucking all the joy out of me.
only time no one gets any commendation is when the group clearly blows, no i don't expect everyone to at least be a 50% raider, but if both dps wear ilvl 490+ and the run takes twice as long as usual while i just know its not do to an afk heal or whatever than you'll either have to be the most awesome person to be around at or well, no one gets the commendation.