There are no visible differences at a UI level. Or at least there don't have to be. Many major third-party apps including all of the Adobe Creative Suite applications and Microsoft Office are Carbon applications. Most new-ish Apple applications and many smaller third-party applications are Cocoa.
As an end-user you should not be concerned at all. Carbon and Cocoa are simply 2 different ways for programmers to get the same out. Some will like Carbon, the rest of us know we're right and use Cocoa 😛
This is highly unlikely as there are a number of industry-standard applications that require the carbon API (Adobe, Quark etc). I suspect that Apple are aiming simply to re-write the remaining parts of OS X that use carbon to use the cocoa API instead.
Don't get me wrong - carbon IS going away, but it's a little too soon - Apple would be shooting themselves in the foot.