Comment on WordPress Hacks by Mark.
If you do not want to go MU you can BuddyPress without MU, just the SU install. Therefore, users can have messaging features etc without the ability to create their own blogs, rather just profiles.
There is also a plug-in called Mingle. It is simpler but It also makes a site a community site.
Community sites are the ultimate dream, but they will only work with some ideas. :) To get people to sign up and fill out a registration often a disincentive.
Your UK Election site is a stellar. It even out ranks the BBC! I will be looking forward when you have ‘election results’ pages maybe even with some maps of the UK, or projected results pages.