I too don't have racks or fenders, don't wear anything special for my commute (aside from a helmet), and carry a waterproof laptop backpack.

My bike's a hardtail MTB, but (for my commute anyway) I ride it single-speed on the big ring x the smallest sprocket (for the workout).

My commute's a mostly flat 4.6 mile (one way) mix of road, parkway, and trail. And I do take the lane if I need to (most often when there are cars parked on the street).