Slushy/slippery conditions are the worst, I have to say. Rain is wet (duh) and cold, cold is just cold, but the intermediate cr@p around freezing is the worst of both. And salt eats your bike. That said, you might be surprised at how many commutes are quite nice in winter, and how just a small handful are in bad conditions. Commuting daily you get into a different mindset and dress for bad weather anyway. I'm never as cold as when I wimp out and take the metro, dressed in wimpy thin office clothes.. 
If you can afford them and you do get ice, I'd recommend at least one studded tire, in front. It gives you much needed traction for steering. It doesn't matter as much if your rear tire slips, you just end up on your feet. You'll want fenders, and you can tack on half a plastic bottle to extend them if they're a bit skimpy. And if you can, hosing your bike down gently, leaving it somewhere warm to dry, and relubing the chain at least once a week, maye even daily, helps greatly against the salt. And you need LIGHTS. The small blinky diode ones are cheap and quite visible, get several and make sure you can be seen from several directions.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett