As expected, the release candidate includes much improved tab functionality, from the new square-ish visual elements, and moveable tab bar elements to increase customization. It also includes features that enable a user to refuse tracking from advertisers.
It also promises to fix a number of remaining bugs and minor annoyances that plagued the Beta and developer previews of the browser. The newer, more stable version of the browser needs to be pretty picture perfect if MS wants to regain its market share lost to Chrome and Firefox since IE8 started faltering.
Winrumors got its hands on the RC version and has posted up a complete review of the new release which you can read about here.
A couple of the big new features include tracking protection and an ActiveX filter option. The new tracking protection is a privacy feature that allows consumers to turn off being tracked on the web. Winrumors writes, "Internet Explorer 9 RC offers users a new opt-in mechanism to identify and block many forms of undesired tracking such as cookies, web beacons, advertisements and trackers."
The ActiveX filter option is exactly what it sounds like; it allows you to filter ActiveX controls during your browsing session. It's another security enhancement brought to the platform that will help protect the end users.
Finally, it appears that IE 9 RC will score 95/100 on the Acid 3 test and the build number is 9.00.8080.16413.