Tactful ways to inform people that they're playing badly?

[Vnolan]

I keep reading about this stuff on the forums but given how rare I actually see it in game the situation feels a bit exaggerated.

Conservatively, I'd put it at about 50% of runs, and this is for basic stuff like AoE'ing when multiple mobs are present, pulling multiple packs when the tank is getting tickled by small packs, having level appropriate gear, healers throwing out some dps spells when the tank is near max health, simple dps rotations, etc. This is for lvl 60+ content.

You would rather take a 30 minute penality over a 20 minute dungeon run?

Looking at it from the wrong angle. How much time will be wasted by the noob in the next several hundred runs they join because everyone refuses to do anything about it?