I carry:

bear spray
Hemostats
Nippers
fly dry powder (seems to work better after I land a fish)
cortland fly float gel type stuff
4 fly boxes (hoppers, stone flys, san juan worms, and everything else)
net
tippet (3x,4x,6x)
Leaders (2x,4x,6x)
Indicators
nikon s240 digital camera
tape measure/scale... although this is mainly for when I'm bass fishing on my boat. But nice to have if I ever land a trophy.
Clif Bars (Usually one or 2)

Sometimes if I'm going for a hike, I carry a 2L water reservoir in the large back pocket and Lunch... PB&J.

Paul