I think Andrew's "What's new in Python x.x" documents are a good start. What's missing from them?
The migration path, including hints on how to port Python applications from one version to the next, how to work around deprecations, tips on quick ways to find problem areas in the code, etc.
Andrew usually puts more focus on new features than changes which could cause older applications to fail -- which is what you'd expect in a document titled "What's new in ..." :-)
OK. The question is, who's going to write the migration guide? One problem is that often specific migration issues don't surface until after the release is made.
--Guido van Rossum (home page: http://www.python.org/~guido/)