added: Linda Holmes, "'Top Chef': How Did Such A Great Season Lead To Such An Uninspiring Finale?" My answer, which I posted over there:
The constraints on too many of the challenges have prevented the contestants from demonstrating greatness. Instead it's been more a matter of surviving from week to week in cooking stuff at Target, with mediocre equipment, or from other chef's favorites (among others). When given the freedom to just do what they do well -- the Restaurant Wars and Family History episodes in particular -- the show has been great. There just hasn't been enough of it.