Really, people who are nice and stand out get my commendations. Default choosing the tank or healer is not the best approach. I had terrible healers and tanks in some runs....man, even I had my bad days where I healed or tanked like an idiot. Getting commendations for screwing up? Not really. I often commend DPS if they did their job well. You appreciate the occasional bard that knows what those ballads are for...just to mention one example.

Too bad really if you are in a premade group of 2, joining DF to get the rest. Had an awesome Haukke HM run recently. Tank and Healer were exceptionally good but me and my friend could not give any commendations because we were grouped up :/