It's not uncommon for me to finish and exit a raid/dungeon/trial (as a healer) without commending anyone at all. Typically I go into them with certain expectations (the tank holds aggro on trash mobs, DPS uses AoEs when necessary, people stay out of enemy AoEs, etc.) I won't commendate people just for playing decently unless the rest of the party is utter garbage and they're the only other person who is legit not giving me a headache.

A DPS is more likely to get my commendation if I see them actually do mechanics and isn't face-tanking every avoidable thing. But that's my minimal criteria. Unless I'm having to constantly throw out raises/was killed, I don't expect Refresh/ Mana Shift- but when someone is throwing those out liberally (as necessary) I will take note. For that reason, Ranged DPS/Casters have an innate advantage for my comm.

If anything, I'm more critical of healers/my co-healer in deciding if they'll get the commendation, but by default it's usually in their favor simply due to the higher stress/awareness required for their role. But if I get paired with a 2nd AST composition and they don't overwrite my Expanded buff more than once (the 1st one I'll assume is an accident) and also heals in the opposite sect effectively, you'll be hard-pressed to earn that commendation over that other AST.

As a DPS my comms tend to go to the healers though, especially if I was healed through/Rescued/shielded for something I should have avoided/could have killed me. The only other time I can recall commendating another DPS is if both healers were just bad at staying alive/healing and if I can recall said DPS essentially avoiding as much unnecessary damage as possible or constantly having to raise the healers.