When I compare images that I shoot with my 30D against images shot with a friend's 600D, the 600D consistently produces sharper images than my 30D. This is true even when the images are viewed at the same size on screen.
The difference in sharpness is pretty substantial, and even images shot with my Tamron 70-300 f/4-5.6, which produces soft mushy images with my 30D are significantly sharper when shot on a 600D. The difference in sharpness is reduced when using a good lens, and I can barely see a difference when using the 70-200 f4L. Images shot with an ancient 50mm macro fall somewhere in between.
Does the newer camera incorporate any technology that makes images appear sharper?
My best guess so far is that the AA filter in the 600D is probably a lot weaker than the 30D. Is this enough to explain the difference?