Anyone who was actually doing their job correctly/well. I usually don't DF much, except for my daily MS DR run and any Prae/CM runs I may do later in the day for fun/spiritbinding respectively. Especially when it comes to CM, a lot of people who have obviously been in the run before and have gotten higher tier gear, still play very, very poorly. I've had CM runs where I gave no commendations because everyone in the party was just that bad.

My highest rate of commending seems to go to dps usually, simply because they've shown to be smarter than most tanks I get. Tanks and healers are about even when it comes to how many I've thrown commendations at. Socialness isn't big on my list of things to care about when it comes to PuGs, so even if someone doesn't speak the entire run, if they've shown they're good at their class and play smart, they'll usually get a commendation.