For commuting, I personally recommend a rack for panniers and fenders are often a good choice. After that, whatever bike you like riding. Cyclocross and touring bikes often work very well as you can get wider tires and fenders on more bikes than your "average" road bike (whatever that is). If you want something with a flat bar, get something with a flat bar. I would generally avoid front suspension...

Unless you're tiny and climbing lots of hills, I wouldn't worry too much about bike weight (it's total rider + bike weight that matters).