It depends on the role of the DPS. If it's a BRD that plays songs when he needs to, or a MNK that silences attacks, a DRG that used Leg Sweep at the right times for stunning a boss - If I notice that I'll give my commendation to that person, it's the small things. Another good reason to give a dps a commendation is when that player stays alive the longest (on a fight like Titan in Roulette, if I end up solohealing, 1 (or sometimes 2) tank(s) and 1 dps alive, that last dps can have my commendation. You managed to not fall off on Titan, and you never died either! Well done!