A ZDNet article previews some features in the soon-to-be-released Mac OS X Beta:
However, sources cautioned, users won't find a seamless combination of "modern OS" features such as protected memory with the compatibility and user-friendliness of the traditional Mac operating system.
Instead, they said, Mac OS X still retains many of the functions of its progenitor: NeXTstep, the OS Apple purchased in 1996 along with Jobs' NeXT Software Inc.
...is that a bad thing? Anyhow - of note, Java 2, OpenGL 3D and SMP are supported in the new beta.
UPDATE (blakespot): Also of note--no dual monitor support! The text of this preview seems to indicate that it will not even install on a multiple-monitor Mac. Well, if true, this negates OS X beta's utility in my world. It would also be strange, as MacOS X DP4 does support multiple monitors (even DP3 did, but just barely). Hmmm... I'll definitely be keeping my fingers crossed.