Image quality is great with the 5D mkII, but newer models from other makers, even relatively inexpensive ones, are coming close or surpassing it.
The 5D mkIII will need to increase its dynamic range from the current 11.9 stops to about 14 to stay current.
It will need to improve ISO performance with about two true stops (i.e. ISO 6400 will be as good as current 1600)
That almost takes care of the stills side. Stills are already top notch with the mkII.
Now, I know that video is not everyone's game. But put aside personal needs on this one if you are not into it yourself.
Everyone knows that the mkII started everything video wise. The market is huge now, and video is, and will increasingly become, a really important factor for many photographers.
Video on the mkII was partly really great, but it is, to a large extent broken. It has moiré and aliasing that limit it severely. Fixing this will be huge.
Stronger processing (CPU) performance might eliminate the need for line skipping and up the video resolution quite a bit. This combined with a mostly artifact free image will be important for the mkIII.
Perhaps they even throw the new ALL-I codec in there, if they don't feel it needs to be 1DX exclusive. But I do think that Canon knows the 5D mkIII needs to be at the very top video wise.
To all of this we can add wireless flash capability, modern AF-system and higer FPS.
When all of these bits fall into place it will be a really nice upgrade. Bring it on!!