I hate fighting in forests because historically most factions would never do it, they'd seek out a good and fairly open field to fight on. If a phalanx-army attacks you in a forest it's just... weird.

"Don't go in the forest" is well and good, but it's hard to see on the campaign map just what the engine will interpret as "forest" on the tactical map.

Germanic tribes launching an ambush is a different story, though. They're supposed to do this kind of thing.

All in all it's one thing Medieval 2 does much better. No annoying forest-fights there.