First, don't worry too much about weight. It isn't an important factor for a commuter bike.

Make sure whatever bike you get has eyelets (and sufficient clearance) for fenders and at least one rack. Take advantage of those eyelets and install fenders and a rack or two.

Frame and fork suspensions are undesirable for street riding.

Don't cut costs with your tires. Get a high quality, flat resistant tire. Personally, I've had excellent luck with Schwalbe Marathons (far less than one flat per 10,000 miles), though other high quality tires are available. Wider tires tend to provide more comfort, with little, if any, downside for commuting.

Buy a good lock or two (or more). Your bike should always be more difficult to steal than the bike next to it on the bike rack. If you find yourself riding to the same place often, consider keeping a lock locked to the rack in that location so you don't have to carry it with you every time.

Internal geared hubs are better suited for urban riding than derailer gears. One big advantage is that they can be shifted while the bike is stopped.

Disk brakes, roller brakes, or hub brakes are superior to rim brakes for a commuter bike, as they still work very well when wet. (This is very important for the front brake, not so important for the rear brake.)