Chris Wilson from IE talked about the new features.

• Beta1 is all about developer and web authors features, getting the platform ready. A later beta will have user experience improvements
• Predictable Experience for Users:
• Domain Name is Highlighted in Address bar
• Improved Manage Add-Ons Experience
• Improved ActiveX: Per User ActiveX Install without admin, Per Site ActiveX Controls, DEP/NX code execution prevention
• Users Sites & apps must continue to work with a new browser
• Improved User Productivity
• IE 8 is resilient after crashes - the frame continues to run, only the tabs gets reloaded
• Support unlocking the web with W3C ARIA
• Improved Zoom capability (Page layout is preserved)
• Everyone uses lots of web services and it is manual.For this there is Activities now. It is implemented with OpenService. As a service provider you can check for your service being installed, if not prompt for install. (ie8.ebay.com has a sample)
• Portions of a page is important, that's where WebSlices comes. (ie8.ebay.com has a sample). It uses the Windows Feed infrastructure to keep it refreshed.
• Complete CSS 2.1 Compliance
• Generated Content and Counters
• New 'display' values for tables
• Outlines
• CSS 3 Box-Sizing Property
• CSS 3 Vertical Text
• Unbiased CSS Test suites
• All IE8 CSS/JS have a debugger in the box.
• Most of the performance problems were Network Perf is frequently the problem
• In Broadband the parallel connection limit increased to Six from two
• JScript improvements
• CSS Selector API
• HTML 5.0 Offline storage, offline/online events
• Beta 1 is available from here.

Read the complete post at http://www.venkatarangan.com/blog/2008/03/05/Mix+08+Internet+Explorer+80.aspx