Drivetrain: Single speed. Not that I've ever run one personally, but if you're riding in snow then it becomes single speed by default. Might as well get used to it and save your derailleurs the wear and tear.

Brakes: Disc, although I don't know between mechanical and hydraulic (never had 'em).

Bar: Risers work great, and have gotten me through 10 winters now. But this winter I'm running Albatross bars, which so far have rocked just as hard as they did this summer. No experience with drop bars. I guess that my vote would be for Albatross bars, mostly for the company.

Cargo: Rear rack and panniers work great, helping keep weight on the rear wheel for additional traction on the ice and snow. And if your panniers aren't waterproof, you can always strap your waterproof messenger bag to the rear rack on wet days.