On to the questions:
Q: What should I gem for?
A: If you're doing normal modes, match for most socket bonuses, and use mastery/stam when possible. If you're doing hard modes, you should just gem stam wherever possible.
Q: How much hit/expertise should I have?
The melee hit cap is at 961 hit rating, and expertise soft caps at 781 rating (unless you're a SoT using paladin). Vengeance makes it so you don't really have to worry about this too much, though.
Q: I keep dying on _________. What can I do?
A: Keep up attack speed slow debuff and attack power debuff. Take any survivability talents you may have omitted. If you're doing this, and still having problems, show us a log. We can't help you otherwise.
Q: I'm a warrior and I have problems with single target threat?
A: What's wrong with you? Oh my God, you don't have incite specced.
Q: Wait, what is that, it looks like a Zebra and an Ostritch. It's coming towards me awfully fast.
A: GOOD GOD MAN, RUN, RUN. GET TO YOUR TRAINER AND SPEC INCITE!
Q: Why is it that Tank X is so much better at this fight than Tank Y?
A: Tank X is probably not all that much better. But, in any case, the real answer is because tanks aren't balanced against each other. In theory, they're balanced against encounters. But that doesn't really happen all that much either. Emerald Dream will fix it.
Q: Hey, which trinket should I use?
A: Generally, your best practice is to not use avoidance trinkets. You should probably use at least one stam trinket. If you'd like to reduce your overall damage, a mastery trinket is probably your best option. There's one with an agility proc from a rep vendor, so try that.
Q: Which meta gem should I use?
A: If you're a druid or DK? 2% armor. If you're a paladin or a warrior? It gets tricky. The 5% block value one is currently only giving 1% block value. This won't be fixed until 4.1. So just use the 2% armor one until then.
Q: I'm having threat problems on the pull. Any ideas?
A: Have you told your DPS to stop being retarded monkeys? If that doesn't work, make sure you're opening with Tricks/MDs, using your threat cooldowns on the pull, and not missing any GCD's due to movement. If you're running at low levels of hit/expertise, try to pick up a weapon with some on it, or one with a threat enchant that you switch to your normal weapon after you have some vengeance.
Q: Hey, what's up with Vengeance? It's such a big part of my threat, but I have zero control over it.
A: Your vengeance is Cinnamon Surprise flavored.
Q: No, seriously. How did this get in game like this, did no one say anything about it?
A: Honestly, we're all a little sick of talking about it.
Q: NERF TANKS, WTF BLIZZ. Tanks do more DPS than me and just because they're getting hit. I'm A PURE DPS BLIZZ, I should do more DPS THAN THE TANK.
A: You're probably bad at this game. L2p.
Q: I don't get why DKs are so strong. They have the most health, the most armor, the most damage reduction from stance, the best mastery and great cooldowns. Why are they so good?
A: We don't know either. Oh, and some DK will say that they aren't that good on one fight that the tank never dies on anyways and claim it justifies everything.
Q: Wow. GG Blizz. I'm a longtime warrior tank and I just got REPLACED by a DK in my raid. WTF?
A: You probably weren't that good of a tank. Also, I doubt you got replaced because of class reasons. Probably because you're a little bit whiny, and you missed the last three raids.
Q: Reforging, how does it work? What should I reforge?
A: This is a bit complicated. Generally speaking, the best items are those that give Avoidance/Mastery for the three plate tanks, and Crit/Mastery for Druids. If your piece has avoidance/avoidance, you should probably reforge to Mastery.
If you're a druid, you should always have either Crit or Haste + mastery. Reforge the second stat, either crit or haste to dodge.
Any more questions that I can answer?