I try to give a comm every time I get involved with a duty if possible!
I give them based on how they did with their party role--regardless of what they're playing, and how they acted. Nice people doing good jobs deserve recognition, imo. Doesn't matter what class you are, as long as you're respectful and do your part as best as you can. Having a nice outfit is an insignificant bonus imo (seeing as glamour doesn't come until level 50 anyway) but honestly I'm going to be more prone to weighing your merit based on how you did and how you acted.
If someone's an unreasonable jerk then the odds are I'm going to ignore them, not give a comm and move on with my life (though I do try to give people fair treatment and the benefit of the doubt.)
As far as part performance goes, one has to bear in mind that supernew players run dungeons too. While it can be slow and sometimes a little frustrating to deal with depending on the situation, we all started somewhere. >.>