1) All versions of 1.6 and 1.7 automatically upgrade to 2.5
2) Things that used to work still do
3) New things work as advertised
4) Your favourite extensions for 1.6/7 install and work as expected on the beta
If you develop extensions (components, templates, plugins, etc), please take some time to test your code against the beta over the next few weeks and report back any issues. The less surprises end users have the better (for you and Joomla as a whole). The Joomla Platform has been upgraded while we are confident it's completely backward compatible, it's likely there are a few glitches.
Please ensure you test installs, usage and uninstalls with Site Debug on and Error Reporting set to maximum in Global Configuration
This is also an important release because this is the one people have been waiting to lock into before the 1.5 end-of-life passes, so it's worth your attention particularly if you haven't worried about 1.6 and 1.7 till now.
Finally, thanks to all those that are making 2.5 happen. The new features look great!