While Zope 2.7 is still not out yet, the milestone plan for Zope 2.8 has been posted online. There have also been talks about Zope 2.9 floating around recently. Near as I can understand them, these are the differences:
Zope 2.7, among other things, brings a long overdue and very well done configuration and installation system. It's also officially "blessed" to work with Python's 2.2 and 2.3, although you can't store new-style classes (define in Python 2.2) in the ZODB object database. Nonetheless, just having access to non-persistent Python 2.3 goodies will be very welcome. Zope, unfortunately, lags behind Python releases, mostly due to a couple of reasons. The 'Extension Class' system, which allow for Python code to subclass from C base classes (which Python now fully allows with its new-style classes introduced in Python 2.2), makes some internal changes and tweaks to Python (as used by Extension Class subclasses and instances) to support Acquisition and Persistence. There's also the security issue. Security is a very strong piece of the Zope story, and the amount of new features (iterators, properties, etc) in Python 2.2 and 2.3 (or any major new version) can introduce potential security holes. Zope 2.6.3, released recently, and the latest Zope 2.7 beta are results of deep security audit to ensure that the new Python features would be safe to use.
Zope 2.8 looks like its primary focus is to bring in recent ZODB features and to turn Extension Classes into New-style classes. (New style classes in Python were partially written so that Extension Classes could die). The primary importance here is so that migration work between Zope 2 and Zope 3 can really begin.
Zope 2.9, as far as I can tell, is going to be the release that starts bridging the two different universes of Zope 2 and Zope 3.
My main hope is that the scopes and deliverables of Zope 2.8 remain as seen in its milestone plan so that it doesn't suffer from the schedule humor that has plagued Zope 2.7. And my other main hope is that all of this delay for Zope 2.7 is from focus and care towards making a solid release.