Been playing around with IE8 RC for the last couple days and am pleased with the upgrade from IE8 Beta.
The features are well publicized on IEBlog: Overview of Platform Improvements in IE8 RC1; it’s not really the web slices and accelerators that are of interest.
Two favorites thus far:
1. Debugger/Profiler
You no longer need Visual Studio or to download a script debugger separately. It has all the features you know and love from IE Dev Toolbar along with a decent script debugger/profiler.
2. XDomainRequest
Will be interesting to see how this pans out.
Other notable include the security and performance improvements since Beta. Perhaps this will help recover some of IE's lost market share?