Not so long ago, I asked this very question in a poll, so it was nice to see this excellent forum post attempting to answer the query "which class should I roll to tank?" Jakalope of US Wildhammer-H goes over the strengths and weaknesses of each of our three current tanking classes (Warrior, Druid, and Paladin), and gives some reasons why you might want or not want to play them.
It's a great post and you should really read it if you're at all interested in the tanking classes and comparisons of them, but here's a summary (note, this is what he seems to be saying, not my opinions):
- Warrior: takes a lot of concentration, needs many skills, gear-dependent, has several "oh !@#%" buttons; most difficult to play well. Crucial MT for progression raids.
- Druid: easier to gear, fewer skills used while tanking and thus easiest for straightforward fights, less versatile while tanking, but can switch roles mid-fight. Not a great MT for 25-mans, but the best OT anywhere, and a strong MT for 5- and 10-mans
- Paladin: Best AoE tank, hands-down. Need to be able to watch cooldowns; need multiple sets of gear for tanking; best 5-man tank; hard to level.