It can also be the 5D's coarser pixel pitch.
After digesting all the information I have come to the conclusion that this may be the core of the issue. Here's my thinking:
Supposed advantages of larger pixelbins:
1) more dynamic range,
2) less noise,
3) better per pixel contrast.
1) more dynamic range
From everything we know this is only marginally true. Using a middle-gray exposure it may be somewhat relevant but certainly not earth-shattering. For a correct ETTR highlights exposure, the difference obviously resides in the darktones where it becomes obscured in a discussion about how much noise is acceptable.
Apparently the difference becomes smaller at higher iso.
For an 8mpx to 10mpx jump it is completely irrelevant.
2) less noise
While true, the benefits are not immediately obvious, and can possibly be harvested only through higher bit A/D conversions. For 5D vs other brands, I believe there are too many design difference for meaningful comparison. For 5D vs 20D comparison, the 5D high iso performance is remarkable and the size of the 20D sensor components may result in more secondary irregularities such as stuck pixels. For an 8mpx to 10mpx jump, it is irrelevant.
3) better per pixel contrast
Seems certainly a decisive factor, especially in a 5D vs 20D comparison. If it is the source of the fringing, then it would certainly account for part of the subjective, easy-on-the-eyes, butter-smooth images from a 5D vs a 20D. However, for an 8mpx to 10mpx jump, it is hardly relevant.
All-in-all, I think that the smaller components in an APS-C sensor vs a FF sensor do create some system/design specific problems that are compensated for in data processing, but for an 8mpx to 10mpx jump I think that none of the advantages is remotely significant. Far more significant seem the additional components such as the AA & IR filters and microlenses.
Obviously, even more significant are the interchangeable lenses and Photographic technique used. In that respect it remains a kind of ironic paradox for an entry-level designed camera to out-resolve the more affordable lenses.
I believe that one of the reasons so many heated discussions exist on this subject is that there may be a relevant difference in large pixel-bin and small pixel-bin sensors, but these are compensated for in data processing. So, the net result is that images do not contain the usual visible noise differences that people look for, nor do the technical graphs show any significant differences.
But I believe that people certainly can experience the differences between an image on the very limit of reasonable processing vs an image that is butter-smooth from the capture stage. Whether that difference truly translates to the expenses required, remains debatable. I personally think it doesn't, not in the least because of the decreasing quality in image reproduction of the past 15 years.
A 1-series with 26mpx will NOT produce those butter-smooth pixels within the next 3 years, but the resulting images will most certainly be accepted as the quality point of reference for the industry.
Even so, I would still consider purchasing a 5D if I can afford it. Not because my photographic skills justify it, not because my imaging needs require it. But there is something magical about butter-smooth pixels which then result in buttersmooth but extremely sharp images, which is simply irresistible.