Just to jump in real quick and provide what I hope is an extra clarification.
There is a substantive difference between the framework being feature complete and the entire Joomla install being feature complete.
The importance of the alpha stage has little bearing on the user "view" of the project. This release will signify that the API, and ONLY the API is feature frozen. This means that it is the earliest point where a third party developer can start developing on the new API and expect it to remain stable. Does it mean all bugs are worked out of the API? no, though we hope there are few. It simply means (for example) that if you want to set the title of a page... then you will from here on out call the same method to do it.
$document = & $mainframe->getDocument();
This is significant for third party developers because they now can rest assured that if they use any of the API in their extensions, they will not have to rewrite them because we changed something. Also, it signifies that we have added all the API functionality that we going to add for this release. This lets the third party developers know that if it already exists, they can use it... we won't take it away... and if it is not there... they will have to write it themselves.
From the user "view" of the project, this release does not mean much because the user doesn't interact directly with the framework (API). The user interacts indirectly with the framework via the core components. The time between alpha2 and beta is what we have allotted for refactoring core components. You can expect lots of commits and lots of changes as we progress through this stage. One of our main goals for this release was to move as much core functionality as we could OUT of core components and into the API. One example off the top of my head that I did just recently was move the user activation code OUT of com_registration and into the JUserHelper class. What this means is that any component can now activate a user if necessary by calling one method in an API class instead of having to either rewrite the code or even worse, including the com_registration file in their extension. By moving to a more framework(API) [drive] system, we are creating much more flexibility and possibility for third party developers.
Alpha2 = Release targeted at 3rd party developers ... users can go back to their regularly scheduled programs
Beta = Release targeted at the entire community ... everyone test please