There are plenty of good posts on this:

Gray Knowlton’s post goes into a lot of interesting detail about some of the under the covers changes and the Office Sustained Engineering team blog is a font of knowledge.

What is most interesting to me in SP2?

• You can easily uninstall this service pack with a handy uninstall tool
• In addition to ODF and PDF support, the document format converter is extensible
• Outlook is now faster… making it easier to handle all that spam you get from facebook and twitter.
• Performance enhancements in SharePoint
• Caching client credentials when SharePoint is in Forms Based Authentication Mode
• SharePoint Support for IE 8, FF2.0 and 3.0