The first few dungeons it isn't a problem. But getting a tank in a level 50 dungeon with less health than the healer is a problem. Even if that healer is synced from 70 that makes it way harder then it should be.
I'd say anything above 30 should have a ilvl for dungeons or such. So people can use the items that give exp bonuses below level 30 still.
Even still it should near the level required to enter the dungeon.