Here is a short list of the things that I think will be present in iOS 5. The correct featureset will be announced at the 2011 WWDC.

1. Improved notification infrastructure, i.e., like the jumping dock present in Mac OS.
2. Storage of contacts/documents/mail/calender events/books/purchased songs and videos on the cloud with no intervention and no limit
3. Improved support for exchange and corporate infrastructure
4. Group messaging
5. Dynamic homescreen tiles
6. Universal search
7. New animations for common actions
8. Support for at least 10 more languages (I would say 20 new languages)
9. Improved iTunes/iPod experience
10. Improved lock screen
11. Font/kerning updates
12. Improved accessibility
13. Maps improvement