1. You will lose the look of sense email (conversation, favorites, unread, attachment will be gone). If you are using imap you will need to set it up manually since it will automatically pick up pop3.
2. People turns to Contacts. FB picture doesn't update in contacts like it did with People (atleast I couldn't figure it out when I was using CM6). This will also be the case with Launcher Pro People widget pictures.
3. You lose sense dialer and you get AOSP dialer.
4. You lose drag down to unlock and get vanilla slide to unlock.
5. SMS you lose the quick attach button for picture, video, etc.
That's all I can think of for now.
With Launcher Pro with Plus you can get back Sense like Facebook widget, People, Bookmark, Calendar, Messaging. To get back Sense like clock and weather you can install Beautiful Widget, Fancy widget, or Weather & Toggle widget.