Extension Manager & Updates
I just read that Mark wants to strip down the core as much as possible and leave any extra functions to plugins. Excellent!
BUT ... ;-)
I like to suggest an extension to the extension settings on the admin pages:
Please show all available (or as many as possible) plugins directly in this section - so people at least find the most important directly IN THE ADMIN section instead of another (official) repository.
Download & Install
Even better: a one click download, install and activation prodcedure - instead going to some other site, download it, ftp upload, admin interface and activation.
A routine that shows if updates for plugins or the board are available. Many phpBB boards got hacked because admins didn't know anything about new versions or patches. IPB has an version detector and direct download link.