Seconded. What I notice most when I'm on the verge of burnout is a lack of capacity for stress. Say when something breaks first thing in the morning - under normal circumstances, it's cause for some grumbling and maybe frustration at the time taken out of your day. When burnout is at hand, I'm just out of my mind. I don't have the tolerance for even the minor stressors, and nowhere close to my normal patience for the slow, thoughtful work necessary for top performance.

Burn out - for me - means a shorter temper and less productivity.