Graphically, mw2 was superior. Other than that, I can't find a whole lot that was better than what Black Ops is giving us. I agree that it is a bit disappointing how little challenges play in to the multiplayer experience, but they never really did in previous cod's and it seemed that in mw2 it turned more in to who had been in the better challenge lobby.
I love the gameplay in Black Ops. It is fast paced and everything is generally balanced (with the exception of snipers). The game really feels like they were trying to take it back towards the COD 4 playing style. I am (for the most part) pleased with this installment in the COD series and hope to see 3arc make something even better the next time around (hopefully they can get some lessons on how to make a game graphically pleasing in the meantime

).