From a person involved in a first gen next-gen title, pushing the envelope is hard. We are all stuck on PC, without real hardware to really see whats they are good at (specs and alpha kits only tell you so much).
Also its takes considerable effort just to produce code, let alone good code. So much of the stuff is very unoptimal, it will get better as it gets iterated on but with very limited time for launch don't expect miracles.
If we take PS3 or Xenon, I expect a 'simple' 2 to 3 improvement in performance from chucking functions like GPU command buffer creation, animation and physics onto other threads. Also the graphics will probably be a couple times faster due to lots of ALUs and fillrate. However RAM limits are going to be hard. I'll predict that putting in a 'reasonable' effect into a launch title should be about 2 times better than a high end PC game today.
We also want more complex games, which mean lots of non hardware oriented devs, that will be THE fundementally limits on performance. Each generation we get further from the theoritical maximum of the platform...
Once the next-gen consoles are bedded in (2nd or 3rd gen) expect more, especially on single platform titles.