We killed the new Doom Lord Kazzak yesterday. We'd played with him a few weeks ago with a small raid to get a feel for the fight but inconvenient server resets last week blocked a kill until now. The fight is fairly fun and a bit nostalgic. Essentially Blizzard took the original Lord Kazzak fight and retuned it to be very difficult with a 25 man raid of 70s. None of his abilities are particularly forgiving anymore, he hits harder than most raid bosses, and he heals much more quickly than before. The loot is pretty fair for the time it takes to learn the fight.
Unfortunately, this seems to be one of the last "good" encounters in TBC. It has almost become cliche to whine about all the problems with 25 man raiding by now. Hopefully Blizzard will wake up and start providing 25 man content as good as Karazhan was for 10 man raids, or Naxxramas for 40 man raids.