Tab Grouping groups tabs that are opened from the same page. For example, on a Google search results page if you open the first two links the two new tabs will be grouped with the Google search results page. If you close one of the tabs in that group focus goes to another tab in that group. Its small, but I really enjoy this feature and without knowing exactly what I wanted while using IE7 and FF2 I knew I wanted something like this. Plus the colors for the tab groups are pretty!
The new address bar and search box makes life much easier by searching through my browsing history for whatever I'm typing in. Other things are searched besides history but since I ignore favorites and use Delicious I mostly care about history. At any rate its one of the things that makes it impossible for me to go machines running IE7.
InPrivate Subscriptions allows you to subscribe to a feed of URLs from which IE should not download content. This is intended for avoiding sites that track you across websites and could sell or share your personal information, but this feature could be used for anything where the goal is to avoid a set of URLs. For example, phishing, malware sites, ad blocking, etc. etc. I think there's some interesting uses for this feature that we have yet to see.
Anyway, we're another release closer to the final IE8 and I can relax a little more.