- You will be unable to access any shared Google Calendars.
- What about chatting and video conferencing? If you use Google Talk/Hangouts you no longer have your contacts list available.
- You lose your transaction history of anything you bought with Google Wallet (apps, storage).
- Comments, descriptions on your online photos.
- YouTube history, playlists, uploads.
- Photos tagged of you on Google+.
- Any service that depends on Google authentication to log you in with.
#1, and to a lesser extent #2, is the biggest thing holding me back from completely deleting my account. Of course, without too much effort I could convince the owner of that agenda to switch sides as well, but I realize that’s not always an option (Google Apps used in businesses comes to mind).