Beta 1 added a media uploader, improved admin bar and fly out admin menus. In Beta 2 they have improved the flyout menu styling, tweaked the admin bar, fixed dozens of bugs and improved the contextual help styling.
The stable release of WordPress 3.3 is earmarked for a November release. We should see more features added and tweaks over the next few weeks. It’s worth downloading if you want to play around with some of the new features though I wouldn’t recommend installing it on a live website.
Link: WordPress 3.3 Beta 2