The new "quest" mobs are rather interesting. Blizzard seems to have added a level in-between normal mobs and elite mobs. The new mobs are named, are the target of certain quests, and have a little exclamation mark beside their portrait.
So far, I've found that these mobs are reasonably challenging (at least for someone in quest greens, no heirlooms, and of roughly equal level). They aren't as hard as elites would be. The quest mobs generally have about twice as much health and one or two special moves. I don't think they do much more damage than a regular mob, the main difference I've noticed is that they last longer, which does make the fight more dangerous.
(Although my lowbie night elf mage got rocked hard by Ruuzel. Death grip + knockdowns on a short timer is harsh against a caster. I'll have to give it another try tonight.)
These quest mobs are clearly pitched as a small challenge for a solo player. A solo mini-boss, if you will, as opposed to elites which are aimed at groups. I think the new mob type is a good addition to the game.