The site says its pretty easy to see why Apple needed to delay the OS: it's still full of bugs:
In two days of testing the build, it's not difficult to see why: while 9A410 contains a plethora of changes from previous builds, there are more bugs than ever, including many glaring ones involving user interfaces.You can check it out over here.
The Mac OS X interface is one area of build 9A410 that has finally changed across the board, albeit the change is relatively minor. Title and toolbars of windows are darker than in previous Mac OS X releases and some buttons have changed in appearance while others are entirely new (the Finder's new QuickLook feature now gets its own toolbar button by default).
Two new utilities grace build 9A410: Directory and Podcast Capture, both of which presumably interface with new features being delivered in Leopard Server. Directory has a basic and straightforward interface but is billed as only version 0.3, suggesting Apple may have a lot more in store for the utility.