This first release candidate's biggest update may be under the hood, with the introduction of an automated "compatibility adaptor" of sorts. The Compatibility View feature, first introduced in a previous IE8 beta version, is designed to help get around sites not properly coded for IE8. It essentially causes the browser to act like IE7 so you won't encounter any issues.
In the past, you could activate the feature when you came across a problematic site. Now, IE8 RC1 has its own built-in list of sites likely to cause problems, and it'll automatically enable the compatibility shift for you when you surf to one of them.
The feature appears to work well enough, but it raises the question of why IE8's standards are such that a large number of sites need a "compatibility adaptor" to be viewed. Do I want to use a browser that is incompatible with so many Web sites?