Well, I like road bikes with skinny tires and drop bars, aero position...but a favorite commuter is a converted hardtail mountain bike. Riser bars to get more upright, a more upright saddle like a Brooks B17, more slick-like tires, rigid fork, kickstand, platform pedals with toe clips (or straps). I can maintain a steady 15mph pace, great visability, very comfortable and great for emergency moves in traffic. I have my office bag/laptop on a rear rack.

I also commute on a road bike, though faster it's not as pleasureable. Each to their own but I found commuting on a road bike is a grind, and every once in a while you are going to get delayed by a road hazard. If you are going to commute by bike regularly you need to focus on the practability factor. Hybrids - they seem a little bit fragile to me.

Commuting on your go-fast bike is like car commuting in a Alfa Romeo Spider; you can do it, with the top down..but you'd kill the car and your sanity over time.

Note: My commuter MTB is a Trek 7000zx, aluminum, late '90s, not the 7000 hybrids Trek made in the last 10 years; including buying the bike used (very nice condition) and all the mods I probably have $800 in the bike.

Here's what it looked like when I got it (pic stolen from the internet):