My only reservations about the instance so far are cooldown-related. It's somewhat alarming how much we were able to get away with by just having our DK MT pop Icebound Fortitude every time something ugly happened. We completely bypassed Vezax's kite period this way. I'm told Feral Druids can do something similar with Barkskin and just having a ton of HP.
Blizzard's done some work on cooldowns already, but that primarily seemed pointed at the "chain cooldown" style of play where there's always something active. Unfortunately, many of the fights in Ulduar operate along the lines of "stable tank damage for a while, then a ton of tank damage for a few seconds, then stable tank damage again". This has a heavy bias toward the DK "low mitigation but lots of cooldowns" style of play, since they can orchestrate their cooldowns to be active during the high-damage parts and still be stable otherwise. I'm just not convinced that style of tanking can play nice with the Ulduar style of encounter design.
Of course, none of this matters too much in normal mode (except possibly Vezax since it does trivialize a large portion of the fight). In the hard modes, however, when everyone's stretched a little thinner, I have a feeling we may see a Sarth3D repeat.