For DPS I watch, especially if I've decided to tank or heal, for the clear speed if it's a dungeon. Also I will be able to easily watch the rotation of a melee DPS and if they're doing things well, I'll consider giving them a commendation. Also for casting DPS, especially in 8 and 24 man instances, if a summoner or red mage is throwing out raises, I try to watch for that, and will commend them for using that utility.

In general, though, I just kind of try to rank my teammates and watch for who performed their job the best from my perspective, and don't just throw a comm to a tank or healer for just doing what their job is.