IE8 RC1: Security
IE8 RC1 also offers improved protection from sites flagged as phishing or malware threats. In addition to warning you when you encounter such a site, IE8 RC1 makes it more difficult to disregard the dialog box and click through. Finally, IE8 has added protection against clickjacking -- a kind of attack that became common toward the end of 2008.
These are all strong steps in Internet Explorer's development and will be good features for current users of IE. Still, considering the comparable protection available for other browsers, I don't know that they'll hold a high enough value to bring any outsiders in.