Here's a few web application features that I think would be fairly useful to have a shared body of knowledge about. Put another way, I'd like to see how someone else designed and implemented them so I can rip them off:
- Invitation systems ala Gmail when it first started. To my eye invitations have a surface triviality that probably goes away when you have to manage the rationing of invites. Wonder what it really looks like inside. Any small web startup could benefit from having such a system
- Flickr's authentication system for allowing third party applications to perform operations, some dangerous, on a user's behalf. Their scheme also supports revocation of permissions.
- Yahoo! style rate limiting, to prevent morons from taking down your shiny new web services API.