I spent 5 years riding northeastern U.S. and Canadian enduros in the 125cc expert class and another 3 in the "B" class before that. All the clay mud I've seen may have made me biased, but I can't understand why anyone would want to go into the woods on anything bigger than a 250, and a street bike at that.
The first rule is to not have to drag the bike out of the woods. Back in the day, that meant a Honda four stroke, and that is what I would still ride. I rode a total of nearly 20,000 miles on 4 of them and never had one not get me to the finish line because of a breakdown
A 250 will easily get you a ticket everywhere except for some interstates, and be able to carry 30+ pounds of luggage with ease.
Suspension, riding position and steering geometry should be closer to that of a motocrosser than a street bike. The lower the center of gravity and the lighter the better. Avoid bikes with fairings, windshields and other plastic "styling cues". As near as I can tell, that stuff is just there to mark the spot where you fell down.
Make sure you can safely lay it on it's side for things like tire changes and repairs. Skid plates are nice, as are engine protection bars, but a narrow single cylinder thumper with nothing sticking out is better. Things like cables, chains, levers, pedals and lights should be easy to get to for repair or replacement at the side of the road.
I have looked at a few "adventure" bikes, and they seem to be designed to be ridden on roads where it would be safe to take your typical soccer mom's SUV, and even then you couldn't make time like you could on a good 250cc enduro bike, especially if it involved any mud. If you want to do long distance touring on paved roads with it, you might want to consider a 450 to 500cc single, but only for trips longer than 1000 miles.
