A mobile application to assist in the job of house/flat hunting. Something that could tie in to RightMove (at least in the UK), access your saved properties, and provide a simple interface to schedule viewings (although I guess this would still have to be a manual process to book viewings, but the app should provide a calendar to store them in). The app could then alert you when the viewing is soon, and provide an in…
The rental market in London is pretty ferocious. I don't think comments could work because you'd always run the risk that somebody has decided they like a place and want out off other people from looking. In the end you'd still have to check the place yourself. I like a lot of your other ideas though and user photos could definitely work.
EDIT: ...but even then, you might not get personal opinions. Tough one!