• There might not be any de-listing of content from the marketplace now. MIGHT. Microsoft says they haven't de-listed anything "yet", and the whole point of the new dash is to make things easier to find—the problem that de-listing games was going to solve.
• Playing from the hard drive still requires games to be in the drive while you play, which means you can't just load all your games onto your Xbox and swap games from your couch.
• Hard drive gaming is mostly for saving load times.
• The current blade-based dash design still exists inside the Guide, and they've ported a lot of the current dash features there so you can actually access much of it from inside games. No browsing marketplace though.
• The new dashboard is fast and fluid, getting rid of the delays that come up when you're switching blades (occasionally) and loading up the thumbnails of your XBLA games. There's also an Apple TV-like reflection under each icon representing games, and the whole thing looks vaguely Cover Flow-like.
• There's a new Quick Launch feature from the Guide (the thing that pops up when you click the Xbox button on your controller) that lets you switch to any game you currently have installed. XBLA games or retail (if the disc is in your drive).
• Avatars will be used in XBLA games, Xbox 360 retail games and Xbox Primetime games.
• Avatars will be available for everyone, even Xbox Live Silver members. If you don't like them, you can still use gamerpics instead.
• "Primetime" games are like gameshows and will be more of a streamed experience rather than a boxed game or an XBLA game. It's a third type of game on the Xbox.
• All of the marketplace content will be listed on xbox.com so you can browse and queue up downloads for them to your Xbox. If your console is on, it seems like stuff is downloaded as you add them to your queue.
• The 8-people party system lets you basically chat with all your friends no matter where you are in any game, and then be able to drag each one through games with you. People who don't have the game the party leader loaded will get a screen telling them to go buy it.