As a DPS main myself, I tend to give other DPS a com (if they didn't drag the party down). If the other DPS is manqo'te, they have to excel in order for me to give them a com.

I feel like tanks and healers get enough coms for simply existing in a party.
Even though I only have PotD as a reference point (I did like 10 level 60 dungeons in total with my PLD), I got the most coms when I was leveling healers and tanks.
Which was strange to me because I felt like I never really did something special.