A solid 2-4 mb pipe is good enough atm given our usage and services provided to us.
There's HD streaming services as mentioned. Then there's the ability to play games from your not so great pc that are running on far more powerful machines elsewhere. The ability do deliver high speed downloads (Bluray quality HD movies and the likes) for rental etc.
Lots of games at the moment that you get on things like steam are 10gig+
You're defining need by what you already use, if you find you have more bandwidth you can do more things and people can provide new services (RIAA/MPAA etc obliging of course)
Ease of access to more bandwidth at lower latency could spark a boom in services not only at the consumer level but at the business level too. Cheap 100mb pipes for all would change the intellectual development and distrobution networks of the country.
One thing to note of course is that high penetration of superfast broadband doesn't == cheap broadband, web space and bandwidth usage in Japan in prohibitevly expensive.