Tip for questing as protection:
You should always strive to have as many things hitting you as possible as long as you can take the damage.
At least in the 85+ zones, I discovered that if I was focused on using shield block that I would have issues where I had to pop defensive cooldowns or rely on focusing down mobs one at a time to get heals from victory rush.
If I use shield barrier instead, and shield block as an incidental rage dump, I am an immortal god who happens to kill things somewhat slowly. If I can find something that drops damage zones that I can stand in and spam shield barrier, I instead am an immortal god who kills things quickly.
Probably the best example would be the rare spawn mogu. If I focused on staying out of the massive cleave zones and using shield block to mitigate melee swing damage, I had some issues with the mob's stacking enrage buff and adds (meaning I occasionally didn't get out of the way fast enough as I couldn't see where the cleave zone ended). If I used shield barrier to eat the first couple of cleaves, I had a nice big vengeance stack and downed the mob much faster.
Bad tanks stand in the fire.
Good tanks avoid the fire.
Really good tanks use shield barrier to stay in the fire as long as they can safely absorb or heal off the damage, then let loose with a massive vengeance stack.
This also can improve your survivability, since your shield barrier scales with the attack power you gain from vengeance. This makes soloing often faster than playing as a dps, speeds up heroic 5-man runs, and can be a huge source of free damage in raids AS LONG AS YOU COMMUNICATE WITH YOUR HEALERS.
If you're going to mass pull like this out in the world make sure you damage everything you pull (especially use heroic throw/throw instead of taunt when pulling at range). If you body-pull or taunt a large group and another player drops an aoe right before you get some damage out, they'll get credit for all the mobs and you'll have to deal with them being stuck on you. Some enterprising folk (read:bastards) will also drag their mobs into your aoe once they've been tagged.
The best way to avoid this sort of thing is to pull every mob within leash range of your current position. Not only do you make it difficult for people to use you to get free kills, but you make everyone else regret not rolling a tank, as you effortlessly keep ten or more mobs swinging on you and your health bar barely moves because of shield barrier.
Also, group with people. You benefit from their dps, and you can use them to gather up even more mobs for you.