Here's a list of what we've found so far, and we'll add to it if and when we find more.
1. Apps. We knew this from the start...you can put your favorites apps within easy reach.
2. Live tiles. Live tiles change dynamically, and third parties can tap into them. There may be live tiles for sports scores, weather, and news. So far, there are live tiles for email and SMS, people, calendar, and others.
3. Multimedia: We recently discovered that you can not only pin videos to your Start screen, but also favorite artists, albums, or songs. If you're pinning a video, you'll see a thumbnail appear on your Start screen. If you're pinning a music artist/album/song, you'll see the album cover. Neat!
4. Places. Find a sushi restaurant in the city you want to find your way back to you? You can pin places to your Start screen via the Bing maps app.
5. Contacts. Call the same three people the most? Pin them to your Start screen. You can also use this feature to instantly stalk your friends, since clicking on a contacts tile will also let you see their Facebook updates if you're set up for it.
6. Photo Favorites. You can pin your Favorites album to your start screen.
7. Websites: This certainly isn't that exciting, as all other smarpthone operating systems can do this, but you can pin your favorite websites (like pocketnow.com, wink) to your Start screen for easy access.
8. Twitter, weather, sports scores, stocks (all should be possible via third parties)
9. System indicators like battery life percentage
10. Links to other documents/files (PDFs, etc)