If you are planning to mostly trail ride on your bike, you should look for a four-stroke bike with hand guards (to protect you from trees and rocks), a skid plate (to protect the bike from rocks), a headlight (for safety during the day and for better vision at dusk), and possibly an electric start (to make it easier to restart the bike if you get stuck in a tricky spot on the trail). You can also look at dual-sport bikes, which are street legal. Depending on where you live, this can be a great way to eliminate the need for a way to transport your bike to off-road riding areas.