Too dependant on service. Sure he can capitalize on even a small chance but on bargain basement rosters where the best players have to carry the load he was anything but ideal.
As soon as you have to move Defoe anywhere around the park he goes from DP worthy to just another solid contributor. The best players on the recent history of the league (Donovan, Henry, Keane, Gio) can impact the game in many different ways.
Defoe is one dimensional. Awesome if you are an elite team - not good if you are a MLS squad starved for talent.