bboard's probably the best resource now. Since we've not released yet, we've not written up any documentation for external users on the changes we've made.
The two major API changes are that acs-notifications is subsumed into an enhanced acs-mail package (rather than support both), and that the "places" package is disappearing, though you can of course use the existing ACS 4.2 version if you want (acs-references, a more general solution towards providing dataset resources, replaces places).
The CR will now allow you to create content in the filesystem as an option (rather than placing all content in the RDBMS). If you're planning on putting content into the RDBMS nothing has changed.
For the most part our changes have been at a lower-level than packages care about, which is why ACS Classic packages that don't depend on acs-notifications and places should work out of the box (you can use the existing ACS 4.2 acs-notifications if you need to and aren't willing to switch to the enhanced OpenACS acs-mail package).
