If a "commuter bike" is one that must carry baggage, be reliable on city streets, and function in all weather, then:

--Rack mounts
--Fender mounts
--Clearance for tires suitable to the terrain, with fenders
--Somewhere to carry a battery for your lights
--Really robust wheels, well prepared
--Tires that resist flats
--Tires that maximize wet traction on pavement (ie, not knobbies)
--Somewhere to carry a pump
--A riding position that allows the rider to see well in all directions, and be seen by other road users.

A lot of bikes can meet these criteria, including a number of hybrids, flat-bar road bikes, touring bikes, mountain bikes, and cyclocross bikes.

And of course you can commute on any bike at all, just as you can walk in any pair of shoes.

RichC