Hi, I'm Merial, level 70 protection warrior. Want to learn to tank 5-mans as a warrior? Read on!
As a warrior tank, your primary threat generation tools are, in order of preference:
1. Shield Slam (if you have 31 pts in protection)
3. Sunder Armor (or Devastate if you have 41 pts in protection)
Pop them whenever they're up, in that order. It takes a little practice to get a sense for how much threat each one generates relative to your healer, but to a first approximation, a single application of one of these abilities will keep a mob off your healer for 5-10 seconds and off an AoE DPS'er for about 2 seconds. Keeping threat on the primary DPS target requires you to be practically spamming these abilities.
Ways to generate AoE threat include Thunder Clap (breaks CC), Demo Shout (doesn't break CC), Bloodrage, and Cleave (only 2 targets). However, none of these abilities will generate a substantial amount of threat, so at some point you really need to start using your single-target abilities--or just leave the job to your favorite pally. Heroic Strike is useful as a high-threat rage dump, but it's not strictly necessary and can be ignored for now.
On each pull, I'd recommend starting with a Demo Shout and/or a Thunder Clap to get some initial aggro on all mobs. Then tab between all enemies and individually apply one of the three abilities above. You may lose the primary target to your DPS if there are a lot of enemies, but that's okay--just taunt it back, and you'll regain aggro *plus* all of the threat they've generated on that target.
On bosses, warrior tanks excel at mitigating damage. You still want to use your high-threat abilities to hold aggro on the boss, but you also want to pop Shield Block whenever it's available in order to reduce incoming damage. Also, make sure to keep Demo Shout and Thunder Clap on the boss at all times, for the same reason. Keep an eye on your health, and if it gets suspiciously low, pop one of your oh-shit buttons: Shield Wall, Last Stand (if you have it), or a healing potion.
When I originally posted this guide to our guild mailing list, fellow protection warrior Causality added the following thoughts:
Shield Slam is actually third on my list of things to do, far behind Revenge and Sunder Armor. Why? It technically causes the most threat, but it's also twice as expensive on a protection-based tank. You can slap on two Sunders with the same amount of threat, and do it incrementally (so you're not losing aggro in the time it takes you to build up the rage to shield slam... I'm assuming that either you have no rage starting out, like for an add, or you've spent your initial rage on Thunder Clap or Demo Shout).
Sometimes, if I'm worried about threat across multiple enemies, I'll intentionally leave one add without any sort of single-target rage, wait for casters to steal the aggro, then taunt. That's kind of like playing with fire, though. The advantage is that you haven't "wasted" any rage, as your threat is set to what the caster earned for his/herself. Thus you end up with more threat on all your targets than if you had been sundering every one. Buuuuut.... if you don't pull it off right, the caster takes it in the face. [I heartily endorse this technique. -M]
I'm also of the opinion that Improved Revenge is vastly underrated. It's a great damage-mitigation measure for trash mobs... I'm spamming Revenge as often as the cooldown allows (with Focused Rage, it costs a measly two points!), and having my primary target stunned roughly a third of the time means less work for the healbot.