I almost always comm someone unless everyone leaves before I can do so. I'm usually a Healer so my comms are between Tanks and DPS. Where I am healing or another role, I'll comm a DPS:

1. If they are my Dance Partner (either as Dancer or another DPS) and they actually dance. I think that little bond deserves a Comm. Maybe others feel that way as the Dancer above mentioned getting a lot of comms.
2. They are new and giving the group a completion bonus. (I'll comm anyone in that situation)
3. They are a sprout and struggled a lot (lots of deaths) but didn't give up and stayed positive
4. They were fun in chat and enjoyable to be around
5. They have the best portrait, and no one did anything particularly noteworthy