The beta realms have been even less stable than normal this weekend, which is a bit inconvenient when everybody is trying to test Naxxramas. Apparently the cause of the crashes has been pinpointed, and it's the Death Knight spell Unholy Blight (specifically Rank 2). It seems that whenever that spell was used, it would bring down the world servers. Perhaps it was mistakenly coded to surround the world servers with a vile swarm of unholy insects, instead of the DK's target.
So the devs have hotfix-disabled it; you will still see it in your spellbook, but it will have no effect if you try to cast it. A worthwhile trade-off for the servers staying up for more than five minutes at a time, I'd say. The other ranks, including the level 80 rank 4 version, are still available. The Storm Peaks and Icecrown zones were also causing issues, and will not be able to be hotfixed, so we'll have to continue to do without them until the next patch.