There has been tons of discussion around Death Knights and tanking, since they were first released. Now the discussion is based around the fact that they can tank in different talent trees. Ghostcrawler made several comments on this in the following forum post.
I'll try not to ramble here.
We like that DKs have three trees in which to tank. It accomplishes a few things. One is it lets a class keep their role while trying a different spec. A mage can switch from Fire to Arcane and still do pretty comparable dps. A prot warrior who wants to switch spec is doing something like choosing Imp Heroic Strike vs. Warbringer.
Second, I'm not sure that the Prot trees for warriors and paladins were huge success stories for us pre-LK. They had a ton of passive mitigation and threat talents that felt necessary, so they had very few talent points to spend on fun stuff (like Warbringer). Druids are in a slightly different boat since they get two roles in one tree.
Third, it lets the DK shift from tanking to dps a lot more readily than warriors or paladins (at least in a world without dual spec). I know what it's like to be the Prot warrior OT desperately trying to out dps the healers on fights where you are supposed to dps. That has improved, but I suspect a lot of players in that situation when dual-spec comes out will opt for a dps spec. DKs have to commit to tanking talents too, but they are more spread out, and they aren't having to give up things like Scourge Strike or Frost Strike to get them.
We don't know what any of this means for the future of older classes like the warrior. With the DK we had a chance for a fresh start. Can I see a world in which Arms warriors tank with a two-hander and Prot warriors do dps with a shield? Sure. Is it easy to get there. Not really. Are players even excited about going that direction? I think the jury is out on that. (And you don't need to turn this thread into whether or not you think that's a good idea.) It gets even more difficult when you talk about the dps Holy priest or the healing Boomkin. Lolz.
Rune Strike won't earn a premiere niche in the game design hall of fame. It does its job, but it's not a particularly fun ability like Shield Slam or even Scourge Strike. We implemented it to solve a problem, which was that DKs are so locked into their rotations that missing an early Plague Strike or whatever could totally botch DK threat generation. We would like to add other DK abilities that center around threat generation, but it's tricky. For one, Heroic Strike aint going into the hall of fame either. Again, it does its job but at some risk to the player (not the character). Second, any new DK abilities we added would have to also fit into their rotation while tanking but not while dps'ing. We can't just make Icy Touch 2.0 that generates a lot of threat. How is that different from Icy Touch? Yet if we make an ability that takes Blood+Unholy or something that takes 20 runic power, it has to fit in naturally to DK cycles while tanking yet be unattractive to DKs doing dps, who have abilities to contend with already.
That's a lot of text, so I'll just say in conclusion, that this is a new class for us all and I think sometimes players don't give enough credit to just how much iteration has gone on in the existing classes over the past 4-5 years. We've tried to make up for lost time with the DKs, but they still have a long way to go.
The good news is, players seem to enjoy tanking with them. :)
You can find the whole thread here.