IMO, one of the most important aspects of tanking is dungeon knowledge.
Tank classes are usually fairly easy to play mechanics-wise, but the difficulty comes in the fact that you'll lead the party.
Players always tend to follow the tank. (and with good reason) Make sure you know what the mobs do, how you should pull groups, whether you need CC on certain packs, etc.
Know the tactics!!!!!
The fun thing about tanking is, once you're used to it, and you've done all dungeons a few times, you'll move fast trough them, and tanking it all becomes like a big dance (at least that's how it feels to me). When I'm tanking in MMO's, it feels like I'm playing with the mobs, especially when playing a class with aoe stun/cc (warrior tank in WoW for example). Pull them together and position them right where you want them to be, aoe stun, kill, repeat.
Once you get the hang of it and you feel comfortable, you'll start enjoying it immensely