Along with your feet, the part of your body most sensitive to the cold is your hands. But, just any old mitten won't do once you hit 60 mph (96 km/h), and the wind really starts to cut through any of your clothes that aren't up to snuff.
So, what should you look for in a good riding glove? First, make sure it's windproof. Anything else will be pretty much worthless on your rides, and keeping that cold air off your hands is of the utmost importance. Second, look for gloves that have some sort of clasp or elastic at the wrist so you can close them tight around the long sleeves of your jacket or base layer. Again, this closure is to keep the wind out. Finally, look for insulated gloves that will help keep you warm throughout your ride.
If you're finding that no amount of insulation or wrist clasp is helping, you can always try a somewhat unconventional route. Before you put on your riding gloves, slip on a pair of common latex gloves like those you would find at a doctor's office. The latex won't breath, and thus will keep the warmth of your body close to your skin where you need it most.