Following my foray into the realm of role play last post, I thought it time to follow up with another Nerdicle!
Virtually all 'hard core' raiders are aware of some of the sites where they can go to learn about their class. No, we're not talking about the official forums. I had to force myself to go there just to link it for you. Sure, there's an occasional diamond in the rough, but it's too painful to wade through the junk just to find the one or two threads worth reading.
Where to go to find useful (although not always Gospel) information about playing your class?
Obviously, even if you didn't know about them before, I've talked enough about Elitist Jerks for you to check them out by now. They do a great job of taking a very numbers-based approach to maximizing performance - even if they're blindly focused on maximizing single-target damage in tank n spank fights most of the time. The DK forums in particular are organized well. Each talent tree has their own thread, and the original poster usually tries to maintain a long summary 'state of the tree' post on the first page so you don't have to wade through dozens of pages to get the basics (unlike the resto shammy forums when I dipped in there - I couldn't find anything without extensive digging).
Dual Wield lovers post in this thread: both frost and unholy specs are discussed there. This is the de facto frost thread since 2H frost is essentially dead (the few hangers-on still post in this thread).
Unholy 2H is discussed here: Scourge Strike being back in, and tons of speculation about how the 3.3 changes will affect it.
Blood's thread original post hasn't been updated recently, but the only real change since 3.2 (or even 3.1) has been the Glyph of Disease spec I discussed recently. There's still some good discussion currently going on there, however.
EJ also has discussions of various simulators: enterprising Wowers have put together their own programs to help determine optimal rotations/priorities and to figure out things like, say, if one Scourge Strike is superior to an Icy Touch and a Plague Strike. Kahorie's simulator has been around a while, and the Team Robot Simulator just went public today.
There's threads for tanks (and DW tanks) as well, along with specific topics like uses of Army of the Dead, DK macros, Gear choices, and so on (I won't link them all here since you can go to the root forum and see them yourself).
Obviously, EJ has become a gathering place for theorycrafting, but decent ongoing theorycrafting discussions can also be found at other places like Tankspot as well.
Tankspot is very tanking focused and discusses all varieties of tanks but there is some discussion of dps DKs as well in threads like this one.
Deathknight.info was a promising site that seems to be getting less updates these days, but their forums are still getting some good activity. My experience there has been more opinion than theorycraft, so I don't visit often, but some of the folks there can be helpful. My buddy Gravity has been active there and you all know he's got a very nice tanking site of his own that discusses mechanics and theorycraft.
Certainly, this list is not exhaustive. But it should get you started, and of course Google is your friend. If math is your thing, then you'll love those forums! If it's not, it can be a bit more difficult, but there are enough 'plain English' posts to give clear suggestions. I've found EJ to be amazing and helpful much of the time, but you have to find your own way of implementing their strategies. My own journey to apply theorycrafting to my raid experience launched my blog with the Finding My Niche series: you should to the same! Try the cookie-cutter builds, learn them and then adjust accordingly to your personal experience, raid makeup and approach.