Processors & Sensors
Most reviews, including DPR, conclude that there are no meaningful differences in image quality.
Well, the Pentax K-5 has better IQ than the Nikon D7000 even though they share the same Sony sensor. The D7000, in turn, has better IQ than the Sony A-580. I can think of a few other such examples.
These may not be not huge differences, but are clearly visible using the DPreview comparison too - even in RAW mode.
Then there are other possible tweaks, such as removing the AA and / or IR filters, improving the microlens arrays etc. that may result in huge improvements - Look at what Fuji did with the 12MP Sony sensor used in the D300 and D90.