For comm criteria in dungeons I use this:

If arrive at last dungeon boss within 10-15 minutes (barring dungeons with intrusive progression such as Bardam's Mettle) comm goes to DPS, tank, healer or no one in that order. If last boss is not reached within 10-15 minutes, no comms.

If I have to decide between 2 DPS I comm the one that didn't die. If neither of them died I comm the one that stayed 2nd on aggro list for the majority of boss fights.

If both DPS don't meet this criteria I go to comm Tank if they did wall to wall pulls while using their cooldowns, or if the healer died and couldn't be rezzed and tank managed to stay alive till boss is killed.

If DPS and Tank don't meet this criteria, I go to comm Healer if they healed via oGCDs and conributed to DPS, but get instant disqualified if I see them casting Cure/Physick/Benefic or if they let people die from unavoidable mechanics such as raidwides (if the DPS/Tanks weren't collecting vuln stacks) or cleansable Dooms.

For Trials/Alliance/Normal Raids I don't give comms because the minimal required contribution per player is so low that you are expected to meet it.

For EX/Savage/Ultimate I go with static, so can't comm.