I ride a Specialized Rockhopper, and traded in my knobbies for tires called, "Comfort Kevlar". They are completely slick down the center, with a ridge of knobs along the outside that never comes into contact with the pavement. When I need the traction, it is there, and when I don't, it is not in the way. Also, the kevlar keeps them tough so I don't worry about glass all that much. I definitely go around big chunks of the stuff, but the smaller shards roll right under them without a problem.
Just my 2 cents.
Kat