My little level 17 horde paladin tanked Ragefire Chasm tonight...
and healed (in tank spec and gear!) after the instance was "technically" finished, the main boss dead, when some of the people in our group wanted to finish the quest from Undercity and find the other 3 bosses. That instance is poorly designed. The main boss is easy to get to and most players (especially alliance who don't have any quests in there) go straight for him and skip everything else just for a quick random dungeon reward, thereby ruining the entire point of the instance.
Tanking at that level sucks because paladins only have righteous fury, two judgements (both share the same cooldown), one seal and two taunts. Impatient groups can easily yank aggro off of a paladin tank in a multilple mob pull because we don't get consecration until level 20. I frequently had mobs pulled off me even after taunting when I would shift targets to grab aggro off a different mob that was attacking someone else.
I remember this problem well with my alliance paladin tank. Warrior tanks get their AoE threat generator (thunder clap) at level 6, plus sunder armor at level 10, which they can stack at will on a target providing they have the rage for it. Druid tanks get their AoE threat generator (demoralizing roar) at level 10 also. This is completely unfair and leaves paladin tanks at a distinct disadvantage with the dungeon finder tool being made available at level 15. Paladin tanks aren't given enough high threat abilities to effectively do their role until level 20.
Our taunts seem to be only effective enough until someone else hits the mob with one of their abilities, because if our judgements are on cooldown, there is no way in hell our melee attack is going to out-threat the DPS which usually has no cooldown limitations.
Ok, enough of the bitching and onto the more important stuff.
I have to toot my horn here because I was healing a rogue and a hunter (plus dps pet) through that dungeon and the hunter's pet only died once when we got overwhelmed by five mobs. I had no healing gear and was fully specced for tanking. That means: I KICK ASS. And speaking of asses, I should get mine to bed because I have a scheduled guild raid tomorrow that I need to be rested for.