It's a complete rewrite. A complete refactor of the core. It offers a powerful and extensible templating system, better user access control, content approval, improved administrative control, etc among other things.
There have been massive changes to the Joomla 1.5 codebase since 'Beta'... to the point where you can't actually get the Beta anymore: you can only grab the nightly builds. It's relatively usable by itself, but if you want to use any of the plugins / extensions, there are not many native 1.5 ones available. You would have to enable legacy mode and apparently that's not perfect, either. For what it's worth, I've been developing a (relatively large) extension for Joomla over the last few months. I've been working against 1.5 and have decided to 'backport' the code to 1.0, just because of how much the code is changing.
Easiest thing is to wait untill it becomes stable and then go on for it. The 1.5 sure has an eZ Backend.