All are good points and I'd like to add one more:

Gearing. I commute on a mountain bike that's been set up with taller gears in the back, but my crankset is still from a regular mountain bike. My big chainring is only a 46 and would you believe the small chainring is a 20? (never use it on my commute :b) If you go into a bikeshop, you can see a noticeable difference in the gearing between a good roadbike and mountain bike of comparable value, especially in the chainrings. You can put Lance Armstrong on my commuter bike and he'd never make it beyond 27-30 MPH on a level surface simply because the gearing isn't set up for speed. However climbing super steep hills is another story.....