+1 for ease of maintenance.
- Also looks nice and clean - less cables, levers, etc.
- Lots lighter. You probably save 2-3 pounds without mechs, shifters, cables and a shorter chain.
- Fitness! You will definitely feel the benefits after a few weeks riding a single speed. When I ride my geared bike now I ride more smoothly, with less gear changes, and a lot faster - especially up hills!
Then............get a fixie and really feel the benefits!