For me, it would be pretty much everything. I don't use the search field, I don't use the bookmarks, I use an online RSS reader, I don't print-preview and print web-pages, I don't use history, I don't use side-panels. I wish all this could at least be optional.
Many of FF3 features now rely on SQLite database as a data storage system. It's a great idea, it's really fast and all, but if there's a need to store data that way, for a web-browser, maybe there's too much data in the first place ?