I've only been playing my DK a bit, but besides the new rune refresh system, tanking is mostly the same. You still use diseases & DS/HS (although the choice there is survival vs threat) and you use cooldowns like you did before. AoE tanking now requires a slightly different approach with the 'free' BBs but afaik that's about it.
Pestilence is weaker for AoE tanking, but BB makes up for it, atleast that's how it's going for what I've done so ar. (but haven't done any serious stuff yet)