If you want to pvp as more of a tank, then go hybrid. Basically you want to get the crit bonus to thrash in madness and get the shock bonuses in darkness. Then you basically thrash/shock combo, and you're fairly durable.
Yes use taunts, and guard squishy people (like healers).
Well with how you're spec'd now, i'd open with wither to put the slow on them, and then probably spike after that.
I would buy your pvp weapon 1st and not even mess with crystals. It will probably be much better than anything you have.