For me, a commuter bike has to be:
1. light - because I carry a lot of stuff on my commute
2. fast - because I'm in always in a hurry
3. aerodynamic - because it's quite windy where I am.
Having said, I use an aluminum roadbike with drop handlebars as my commuter. As far as accessories, I use a blinking light and a headlight when it's dark.
Cheers!