Among the highlights are customizable Dock applications that seem to replace the functionality of the Control Strip, and the fact that OS 9.1 is the functioning Classic environment. According to Wincent.org:
Weire pleased to bring you the latest instalment in our ongoing coverage of Appleis private OS X builds in the lead-up to OS Xis public release later in March. This time itis "C4K46" thatis on centre stage.
Weid like to welcome all of our regular readers - Mac fans, Apple engineers, website masters, and Appleis lawyers - for this in-depth review of the latest build that weive had the pleasure of seeing.
Unlike several post-4K17 builds that weive seen (for example, 4K33 which we wrote about earlier) this one is a full, bootable install disc, just like Public Beta and 4K17 (other builds havenit all been bootable, but required the use of Appleis "Software Restore" application to place an image of the OS on a target partition. To make matters tricky, the ASR application required was a special version that was not publically available... version 2.1.1).
You can find the full review at the Wincent.org Web site.