Originally Posted by
dp_gonzales
Prot Warrior excels at blocking and deflecting physical attacks. So the question you need to ask yourself is: do you want to specialize and be the very best at the thing you're good at, or do you want to sacrifice that specialization for general thickness? Are you confident enough in your skills to properly link Shield Blocks for maximum uptime, or do you want wiggle room at the expense of uptime?
If you stack Haste as a priority and Mastery second, you can approach 100% uptime on Shield Block. This means you will be blocking all attacks, all the time. If you stack Mastery first and Versatility second, you will be much tankier in general but you won't be able to keep up Shield Block. You'll take less damage from things that aren't physical attacks, of course.
I would consider what content you are doing and what obstacles you will be tanking. In general, the vast majority of fights have you tanking an enemy who is beating on you with constant melee attacks while you avoid standing in purple goo. This is why the common recommendation is to focus on Haste. However, if you were more interested in soloing old content than doing current raids, for instance, then Haste would be nearly worthless as you would not need Shield Block at all.
It's worth noting that Fury loves Haste and likes Mastery, and Arms loves Mastery and likes Haste. Going the Haste/Mastery route will help you towards gearing either off spec while focusing on Mastery and Versatility will not.