1) Driver size. Bigger drivers can deliver more bass, small drivers have a faster response time. 8-10 inches would be more than enough for a car.
2) Frequency. Depending on your budget, you might not want to shell out for an expensive sub that can hit sub audible bass frequencies. We can hear as low as 20Hz, so look for that.
3) Passive or powered. Passive subs will need an amp to power them, whereas powered half a self contained source of power.
4) Crossover. For max compatibility with the rest of your kit, you'll want an adjustable crossover control.