I've been biking to work in Florida for the past 8 years (5 miles). Personally, I like aluminum mountain bikes w/ no suspension for commuting. I have a Gary Fisher Mako. I like the idea of a hybrid, but they tend to be more expensive. And I ride too rough for a road bike. In general, my goal isn't to make my bike as light as possible, because I'm doing it in part for exercise.
I use a rear rack with a milk crate ziptied / bungeed to it. Again, not the lightest option, but a great way way carry a backpack. And I have rim brakes, which I like because I know how to maintain them. I've never had disc brakes, but they might be great.
I keep my work clothes in my office, and bike in shorts and a t-shirt. Fortunately, I have my own office w/ a hook on the door, so I keep a couple pairs of pants / a bunch of shirts there. Every so often, I'll bring them home and rotate stuff. Personally, I don't find bike commuting (even in FL) that sweat inducing. For one thing, I'm not biking at mid-day. For another, it's flat and I'm not racing.