Inspired by bug 897499 (and other similar reports):

Currently, the Sync confirmation dialog says something like "this will upload your bookmarks, passwords, etc". It seems that this is not clear enough, since users sometimes accidentally upload previously-stored local data to their account. Maybe it would be clearer if we showed actual counts:

This will upload
- 87 passwords
- 13 bookmarks
- ...
to youraccount@gmail.com.

(Bug 880471 is a different approach to solve similar issues.)