"Also in the "old days" standard hiking system was to wear nylon shorts over wool long johns. I actually like that system although it looks really funny. I am also a big fan of gaiters."

This is almost exactly what I wore in New Zealand while hiking much of the time, except I had on Capilene instead of wool. Over that I had a pair of surf shorts and wore tall gaiters. It is what BearPaw, an old member here called the "Peter Pan" look. Funny looking or not, in cool, wet weather, it works great. Once I stopped, I would pull on my fleece pants. I also had a lightweight Gore-Tex 2 piece rainsuit, but it didn't work all that well.

Jeans are the worst of all worlds-heavy, won't keep you warm at all, too hot in hot weather and when wet, as OM says, even in relatively warm weather, will cause hypothermia. Save them for around town.