The XMB is a nice core for a lot of features but why are they not unlocked yet? To mention a few examples:
Selfmade Widgets
The PS3 uses a widget engine based on open source for the information board. So why not give us the ability to load our own homebrew widget's from external mediums or install them like themes? As they mostly use XML and flash files it wouldn't even compromise PS3's security.
Music Visualizations
The PSP has a lot of them, the PS3 only three. Next-Gen multimedia entertainment? Not really, yet.
Universal Messenger
The PSP can use Skype, why not the PS3? The PS3's messaging system is based on Jabber. It would be very simple to allow the PS3 to communicate with iCQ, Google Talk and other instant messenger protocols.
Open File Transfer
Why can we only send pictures with size limitation via the PSN while we would more like to send music files or savegames. If that uses to much ressources like traffic, then P2P would be a solution.
Session Video/Audio Streaming
We have an audio/video and textchat there. Where's the ability to stream videos or music to watch/listen them with friends over the PSN? If it's the traffic problem again, one PS3 can host a server and invite friends.
Ingame Invites
Change the game unique invitation system and do it like for example Resident Evil 5 does. Invitations via XMB, we don't need another invitation way for every game. Keep it easy.
Trophy Sync. & Compare
Every time we want to compare our trophies with friends account we need to sync them again with the server. Why not local check which trophy file changed, than comparing every file online which takes the user a lot of painful time and patience. Do a simple hash and if the hash didnt change since last sync, let it be.
XMB Music
Click the wrong option and the music stops. Why? There's no need to stop the music whily changing some settings or load a friends profile.