Classic Leveling Guide - Commonlands
The Commonlands is actually a pretty decent spot to level for players 10 - 20. If you're in your low teens and without a group try to hunt around the zone line to West Freeport in the eastern portion of the zone. Those with a group can hit up the Dervish Camps or the Orc camps.
There is also a camp of Shadowmen in the southwestern portion of Commonlands that are home to much higher level mobs than the aforementioned ones. You will also find high level monsters occasionally roaming the plains, Hill Giants and Griffons that you'll need to watch out for as a lower level player.