@Nickno How would it make sense to have RCR work in widescreen on only one system if it were possible on both?

The Wii's "widescreen" feature for its Wiiware games simply stretches the original low resolution image of these games across the whole of your screen. When you're dealing with large, blocky NES pixels and more fluid sprites and polygons this "stretching" is barely noticeable (note: there are exceptions to this), but games that necessitate tiny pixel-perfect objects like Retro City Rampage look horribly distorted if forcibly stretched too far beyond their natural resolution, hence why the developers of RCR have opted to drop widescreen functionality on the Wii.

The removal of the 'Challenge Rooms' and 'Character Creator' features is a little more puzzling, for now I'm going to be optimistic and say that this is merely due to the fact that Wiiware places a smaller size limit on uploaded games than the XBLA, making it necessary for developers to cut a few non-essential features outside of the 'main game'.