If they did all the mechanics well and made the most of the kits (as best as I can tell) I'll give them the comm. Things like ranged using Peloton, DNC keeping their partner buffed, BLMs using their isntant-travel abilities or knowing a fight well enough so they place their circle in the perfect spot, DRGs buffing another player with their Eye instead of just using it on themselves, RDM's not casting Vercure to heal people, SMN pet swapping appropriately, etc.

But usually it means the tank and/or healer would have had to under perform, first. Healer not DPSing or not healing when needed, Tank not using cooldowns as needed etc.