From first-hand experience:
1. MSN 7.5 stopped working
2. Pages take comparatively more time to load
3. Takes a bit longer to launch
4. Saves web pages as .mht by default and does not remember your preference
5. When saving pages as Web page complete, it saves them as HTML only!
6. Completely loaded pages cannot be some times saved, gives error message. This web page could not be saved.
5. Cannot organise favourites, or sort by name
7. Lacks the tab to edit page with your editor (eg FrontPage, Word)
Still not convinced, read: 7 reasons not to try IE 7.0 Beta.
Uninstall instructions here.