Fiberoptic is already being used for alot of transport. Most of the problem with broadband is the "last mile" concept.
Getting all the infrastructure in place to get the signal into your house is usually the issue. My guess would be that
this is fairly new to most of the companies offering it and there are always going to be growing pains.
As far as speed that's mainly a scalability issue. If 100 people in your neighborhood are streaming music and
downloading off some P2P client 24/7 then you will probably have slow speeds.
Downtime is an infrastructure issue. Since this is relatively new, they usually don't have multiple redundant systems
in place. This means that when an upgrade is made or a line is cut, your out of service for awhile.
These are just educated guesses, but someone with more knowledge could correct me if I'm wrong.
Edit: I have had cable broadband since 1999 and have had maybe a total of 1 day downtime. My power has been out more
than the cable service. We lost power for 2 days, luckily we were on vacation and it was warm outside. One thing I have noticed
is that the name servers go down every so often, but since I run my own caching name server I haven't been affected by that.