I like his character more than his game. I think that's the big loss.
Comparing starting C's, he's a lot closer to Darko or Gortat territory than D.Howard, but with less mobility (now) than Darko, less shot blocking, less rebounds (a hunch on that one), and perhaps even less offense than Darko.
I also think Kwame Brown seems to get more rebounds, is also a good defender, but (somehow) manages to be even worse offensively (small hands?).
Howard, Bogut, Horford, Noah, Nene, Duncan, Camby, Chandler, P.Gasol, B.Lopez, Okafor, Gortat, Dalembert, Cousins, M.Gasol, Darko, Kwame, Biedrins, Kaman, Sideshow Bob...
Probably not top 10. Possibly not top 15, but somewhere there in the middle of the pack where talent-wise, they're a bit interchangeable?