Look after your knees!
If you are either heavy or strong, I would seriously advise against a fixie or singlespeed. Pushing big gears will knacker your knees, and they don't repair easily!
Gears allow you to spin the cranks at a cadence that suits your level of strength and fitness, whatever the road conditions at any given time. This is the key to fast, efficient cycling.
Gears don't really add that much weight and in my experience, once set up, hardly ever go wrong. I don't see the desperate need to get away from them, particularly not when I see single-speeders heaving around as they try to pick up speed after a junction, or spinning like crazy as I cruise past in top gear...
And this stuff in a previous post about the rear wheel on a fixie helping you pedal... With respect, it is not possible for a rear wheel to help you pedal. Momentum, or gravity, or a passing bus can help, but a rear wheel is just an arrangement of inert matter. A fixie is a fantastic training bike because it forces you to pedal the whole time, plus in the specialised conditions of an indoor sprint it gives an experienced rider tremendously precise control and allows you to save precious grams into the bargain. But an everyday commuter bike it is not!
The fixie/singlespeed fad is great because it is bringing lots of beautiful old steel frames back into circulation, but seriously, geared bikes are much more versatile and efficient.