The Old Forest is inhabited by animals, plants and rogues. There are frequently large groups, or "pits", of similar animal types -- snakes, insects or wolves. These will typically fill an entire clearing.
The fourth level has an additional exit which leads to Lake of Nur. This special exit is found on the southern edge of the zone, rather than the eastern edge.
The 6th Old Forest lore, found on the 4th level, contains a "key" which initiates the Sher'Tul Fortress quest.
Sometimes, you will receive a message upon entering a new level in this zone about a thunderstorm. The zone will be darker than usual (though still fully lit), and it is raining. Air elementals will continuously spawn, though give no XP when killed. Be wary!