Also:

Other tanks - start fight in tank stance, build up as much agro as they want, then switch stances and start doing a whole lot of damage.

Warrior - start fight in tank stance. If you use Unchained, your potential DPS takes a HUGE drop. If you build beast gauge and swap, your DPS takes a HUGE drop. Having to build beast gauge from 0 is a drop in DPS as well. If you do the Storm's Eye -> Onslaught -> Deliverance -> Butcher's Block opener to make sure you start building beast gauge as fast as possible, you risk not getting enough enmity on the the boss. The LONGER you spend building enmity, the more beast gauge you will lose when you swap, and it's just so freakin' painful when you're tanking. Like you can FEEL it inside of you, all that waste. It's ridiculous and not fun at all.