Brushless motors have a variety of characteristics which make the a good choice over brushed motors. 1. Commutation: Brushless motors do not require a commutator and brushes to energize the coils of wire and make the shaft spin. The friction between the commutator and brushes causes wear. http://en.wikipedia.org/wiki/Commutator_%28electric%29 2. Torque: Most brushless outrunner motors can deliver higher torque per ounce than brushed motors. 3. Cooling: Inrunner style brushless motors have the windings closer to the outside of the motor which allows them to transfer heat more efficiently. 4. Fabrication: Outrunner style motors are cheaper to fabricate than brushed motors and require fewer parts
From a hobby standpoint, brushless motors run longer on the same battery packs we use. This is because they operate more efficiently and run cooler. Outrunner brushless motors can swing a bigger prop and use less current to do so. I have flown electric planes since 1978. In my opinion the introduction of brushless motors and Lipo batteries have had the most profound effect on electric flight since it's inception.
Bill Griggs - Makermasters