Worst part about dining at CityZen? Trying to get a damn cab to take you home!! The Mandarin Oriental is squarely in the heart of D.C.'s No-Man's Land. Even if the restaurant calls for a cab on your behalf, it can still take forever for a taxi to show up. (On the other hand: That little butter roll box they bring to the table ... wow.)
Qualitatively speaking, the thing that I found most disappointing about CityZen was that the quality of the ingredients wasn't quite what I'd hope for/expect from a Thomas Keller disciple. At French Laundry, it seems like just about everything on the plate has a Best of Breed-ness about it, from the rabbits and fish and foie and butter to, of course, the produce, which is consistently, breathtakingly great. At CityZen, none of the ingredients jumped out; they were good, for sure, but not great.
But, then, Eric Zeibold isn't wholly to blame on that front. As a recent transplant from the West Coast myself, I have to say that the hardest thing about moving to this area is realizing that this ain't exactly the cradle of agriculture. Eric pretty much said the same thing when he came out to chat at the end of our meal. He talked at length, in fact, about the difficulty in foraging great produce around here. (He'd just sent back an entire shipment of peaches, if I recall correctly, explaining to the supplier that they weren't even close to the ripeness and quality he was looking for.) I do hope he can land some great sources, because the guy can clearly cook.