The big gains - dropped handlebars, especially on the windy days. Decent rolling tires on reasonably light wheels. Pedals and cleats (or toestraps) so you can pedal full circles. None of this has to be a big cash outlay. An older steel sport bike can make for a very good commuter. It is possible that your hybrid can take dropped handlebars and serve well, but that depends on both you and the bike. (Fit issues.) Pedals and shoes are an easy upgrade and can be transferred to any bike you get in the future. (I kept my work shoes at work.)

If you want to avoid the used bike hassle and are willing to spend some more money, look at a bike like the Surleys. (And don't pay attention to I-Like-To-Bike. He is always quick to point out how what we do and think are wrong.)

Ben