As a user that recently switched to 5D2 from 450D, I call tell you that there are no discernible differences looking at the RAW on a computer screen. There is no way to tell if I'm looking at a 5D2 or 450D RAW, shitty shots remain shitty, great shots stay great. As for your observation, I'm guessing that owners of 5D2 have on the average been shooting longer than entry level DSLR owners which leads to "better pictures". The other reason would be more experienced shooters are more likely to be better at PP as well. Again its shooter, his experience and technique, rather than the camera.
The real advantages for me were low light performance for no flash shooting, narrower DOF for portraits and a little more leeway for cropping.
Hi, thanks alot for the advantages portion...
Will take note.
Cheers!