For me, it's a pretty simple priority system:
1) Nice and helpful people
2) People who play better than most
3) The role counterpart (Healer/Tank, DPS/DPS)

If multiple people qualify for the first step, I go down to the second and even the third if they also qualify for the second.
There are two kinds of people who I don't commend on principle and those are rude people and people sporting "sexy" glamours. In such cases, I give my commendations to someone else even if they qualify for one or all of the prior.