My main is a lock, and it's what I've done since well before AQ. Since Cata, I've gotten into tanking and healing a lot as well, though mostly tanking. I co-GM a guild on my blood DK, tank on my druid some, and tank on my paladin. I know a lot of the tools, such as SimCraft, for getting into some of the theorycrafting and figuring out how to best do my stuff on my warlock. However, I'm very curious as to where/how to start with tanking?
I'm mostly concerned with my DK and druid for tanking, because I'm not sure how to figure out what is actually best for me to do, etc. For example, how do I find out what the value of stamina is compared to mastery for my DK? How do I find out how much better it would be do gem all dodge/agility vs. stamina/mastery on my druid or find out what the relative values of either of those are, etc.? I understand how tanking works, just not how to really optimize my characters as much.
I would start by reading the two stickies at the top of this sub forum, and the posts in the "Critical Guides and Articles Library" forum (including Satrina's Evil Empire articles). That should cover most of what you need to know.
It's a learning curve. If you have tanked, even just once, you should know a surprising amount. However, as you progress, you hit he curve hard, and it helps to know where you are.
If you are theorycrafting for the sake of theorycrafting, go ahead and read the standbys, such as the guides here, Theck at maintankadin for pallies, Kanlon's blogs on wowthinktank, EJ's for specifics tidbits like the combat table and such. If you are having some specific issues, might as well ask here after reading such areas, Tankspot is open to all tanks.
The short answer, after 4.1, is stack the f*** out of mastery. There is no downside to doing so, and no Diminishing returns, and no cap that can be reached with current gear. The next level answer, which is the same as with a warlock using simulcraft/rawr/some spreadsheet, is that there is an interplay between stats, and blindly stacking one stat at the peril of another can lead to suboptimal results once you get past heroic dungeons and normal difficulty raids. Even as DPS, Equivalence Points are usually calculated at a certain gear level or setup. If you are not at that point, unless there is a massive difference EPs (which we currently have with Agi/Str/Int but didn't have in WOTLK), you may be working on false assumptions.
For tanking, I remember seeing some some exotic tools to calculate TTL from ToC days when it really mattered (you'll have to dig for them), but tank gearing in Cata isn't nearly as important as skill and knowledge of the fight, as long as you have the basics of gearing which I think you do. Damage profiles have changed, if you stack mastery, you are only helping your physical damage survival, which is your most prominent damage source. Once it gets tighter, stuff like magic damage starts coming into picture, or more likely, just plain old tanking ability (proper Cooldown/raid utility usage, raid awareness to not stand in fire, etc.). Stamina becomes more useful so you can literally survive certain encounter events or bad luck like network issues or player mistakes. Mastery only helps Shield block or absorb shields of DK/bears (which is always useful, mind you). Beyond that in heroic raids, you may start capping your DPS so you may have to play around with raid strategy, and failing that, some threat stats in your gear.
Don't forget, you can become an awesome tank, but at some point you are limited by your healers. If you are dying, and you determine it's not your DPS or your own issues, either your healer skills are lacking or more likely, your healers need upgrades. Back in vanilla and TBC, choice upgrades went to tanks first, nowadays I find healers are starting to get the prized drops first. Raid survival time = Tank HP + Healer Mana Pool size.