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. Your new 5D Mark III has 22MP compared to 12MP of the 5Dc; as such, the new camera will magnify user error and slight misfocusing results, by the camera or by you, the user.
This is a big one, in my opinion. I first had a 300D digital rebel and was disappointed after upgrading from it to a higher-res camera. In my memory, the 300D didn't EVER have focus issues, and every pic was sharp. But the new, more expensive, higher-res camera seemed to have softer pictures. At first I thought it was just nostalgia and the fact that I was a new photographer with the 300D and didn't usually pixel-peep.
I still have my 300D and what I found is that because of the lower resolution, everything looks sharp, it isn't just nostalgia, especially at 100% views. In fact, I have a lens that takes a pretty big MA adjustment on my newer cameras, but on my 300D it looks fine. Even if I force the lens to front or back focus (adjust focus after AF locks) it's hard to tell much difference until it's really out of focus. And every lens, no matter how cheap, just looks amazingly sharp on the 300D.
So while higher-resolution is a blessing, it can also be a curse. It definitely shows things like misfocus, or motion blur from slow shutters, more than something like the 300D. Of course, when looking at full-screen images, these issues typically don't show up an the higher-res camera is going to give you a better picture (even in small print sizes, the 5d2-3 produce noticeably sharper images); but when zooming in to 100% the 300D/5Dc era camera's giant photosites definitely hide issues better, and make everything look sharp even if the focus is off a little or the lens isn't great quality.
Smaller photosites means you're essentially zooming into the picture more, so misfocuses are also more clear. And with smaller photosites, motion blur is more of an issue.