As an aside, note that this backward compatibility is actually a mixed blessing, because it means you don't have to update your modules now, but there will come a time when it is going to bite you.
When new releases take features away, we will issue warnings as a gentle push. When they add features, I don't know why you should use the new features, unless you need them -- and then you have your motivation in your needs.
As a personal example: the MacPython toolbox modules haven't been updated to make use of the GC stuff yet (and that's been there since 2.0, no?),
But the API was totally changed for 2.2, so you're actually lucky that you didn't do it for 2.0. ;-)
let alone the new type system. And these are almost all generated, so it would probably only take a few dozen lines of code to fix them. And the new type system would be a real boon for some of the modules (such as the windowing and dialog stuff), but because there's no real push (i.e. everything still works) nothing has happened yet...
I don't think anything can be done to force you to start using new optional features... Eventually classic classes will go away, but that will be a long time.
--Guido van Rossum (home page: http://www.python.org/~guido/)