My take on why businesses should upgrade to SharePoint 2010 or 10 Reasons to upgrade

1. User friendliness across the board on SharePoint Sites UI. Good video going through most of the most important changes. 
2. Wiki have been improved tremendously (also support all browsers, not just Internet Explorer). SharePoint is effectively turning into a Wiki. if you want more details this video covers most of it. 
3. Ability to have multiple people co-author documents (like Google docs but in the Office client). Working for DOC, PPT, OneNote but not Excel client (only web client). More info here. 
4. Performance has greatly improved across the board 
5. Much better integration with other Microsoft systems/apps such as Exchange 2010, Lync 2010, Office 2010. Video & excellent blog article
6. Much better “My Sites” and social networking for the enterprise. Video
7. Better Business Intelligence features 
8. Better Search 
9. Better Development tools (only for IT Development & Power Users around the business) 
10. Better Enterprise Taxonomy & Metadata management