If a person is both nice and skilled, they are likely to get my upvote. A good 'player' but bad sport (skilled but a jerk) will never get my upvote.

As far as the commendations I receive...

I tend to get the most commendations when I play White Mage in PvE and Summoner in PvP. When people are equally skilled and equally helpful, I notice tanks are most likely to get a commendation (judging from my experience playing with a tank), unless the fight is particularly hard on the healers.

However, you can't give commendations to people who leave really fast... so those who aren't getting commendations should linger for a few seconds to see if they get one.