To be honest I'm usually busy with my role to keep track of every little thing my partners did through the dungeon. There's obvious stuff like how the tank pulled and handled the aggro, stuff that is noticeable if you pay attention like a dps hitting the wrong enemy, and stuff you're not gonna keep track of like who's using an optimal rotation.

In a normal run that went well and where nothing out of the ordinary happened I tend to commend the tank since he/she's the one directing the battle, or the healer if I'm tanking. But certain stuff in the dungeon may make me commend someone else, like someone explaining the mechanics or giving advice for a class, or someone taking advice positively. There's also that case where I went with friends or FC members and the DF person did so bad that no chance I'm commending him/her :P