Every time I use YaST's Software Manager, I wonder if it would be better implemented using Banshee. Banshee's interface has been tuned for usability, both on its own, and what it borrows from iTunes. Software management is a naturally scary operation, and using an interface that the user is already familiar with could help reduce user fear.
Of course, it would just be the interface pieces of Banshee in a new app, you wouldn't actually start Banshee for software installation.
Another feature I would like to see taken from iTunes/Banshee is downloading/installing in the background. Once I hit Install, go ahead and download the application in the background, and install it in the background. I can click on the "Downloading/Installing.." menu item if I want to see what's going on. Most of the time, I'd rather be looking at other things to download.
Speaking of looking at other things to download, why not help me find apps I want to install? We have the openSUSE build service chock full of applications for users, but they're all hidden behind a terrible search-only interface. We need to give the user a way to browse applications and find cool things to do on their Linux machines.
App stores are all the rage, as they give users a way to browse and find new apps, guided by category browsing, "front-page" recommendations by editors, and ratings by other users. "Store" may not be the right word for us, as everything is FOSS, but the idea is the same. One key would be to only show things that are user applications. The user isn't interested in finding things like perl or x11 or gstreamer.
On the same topic, I think F-Spot could benefit from a Banshee interface as well. It's already pretty similar, so code sharing would be nice. Are there other apps that could benefit from using an interface that the user is already familiar with? Monsoon? Nautilus?