Its completely situational, though a tank is more likely to get a commendation from me they are also more likely to not if they derp since I catch that faster than I do with dps.
If a dps does something which I see and is awesome they are incredibly likely to get a commendation, for example the monk dropping mantra at the perfect time for my aoe heal just makes me fall in love with them; also if I derp with my mana consumption and a bard actually notices me as a scholar derp up and using a ballad will get a commendation and an embarrassed thank you.