Gearing - Most motors cannot drive wheels directly. They run too fast and have too little torque. They must be geared down. In general the more you gear the motors down the slower they go but the more torque they have. You can gear them down with gears, chains or belts.
Gears are more reliable, belts are the cheapest and easiest but can slip.
Reversible - It is possible to have a robot with no reverse but it will be harder to control. Make sure your motors can run in reverse. Many car starter motors cannot run in reverse unless modified. Voltage - The voltage and current of your motor will determine the ratings for the rest of the electronics in your bot. Bigger, higher voltage motors require bigger and more expensive batteries and controllers.