I seem to get more comms as a dps than a tank a lot of the time. I don't think I'm doing anything wrong either, though I'm sure I could still use some more practice in the new Eden stuff. If other dps players are like me when I'm playing a dps I will typically commend my fellow dps so it makes sense. I know some people just don't bother, too. But yeah, my standards are usually as follows...
1. I like the person I'm commending
2. The person performed a pretty impressive feat of strength like saving the raid
3. The person was obviously outperforming others
4. Failing 1-3 and granted I remain neutral, I will just pick a person and commend them
5. The person happened to be the only person left I could possibly commend
If I don't like anyone, or if the trial/dungeon/raid was more difficult than it needed to be I will just leave in silence without commending anyone.