To throw in my 2 cents...
Some do relate anesthesiology as boring while they're observing, yet incredibly stressful when doing (especially so without adequate supervision).
Most of us, at some point in med school, learn to deal with at least the beginnings of a patient going down the tubes on the floor. This is considered a "stressful event".
In the OR, if it's your first month on your own, and your training program doesn't offer adeuqte supervision, imagine the stress of slowly watching your pt's sats drop? Or their ETCO2 starts slowly falling off or increasing? Pt becomes tachycardic despite no known significant blood loss? Epidural and/or spinal fails to set up well? Big vascular cases, when the ansurysm ruptures? Bronchspasm? Laryngospasm? 400lb pregnant women who forbid spinal/epidural and now need an emergent c/s? The healthy patient who, for no reason at all, just drop into true, verified asystole during an elective case and don't respond to the first round of epi and atropine? These things seem to happen faster in the OR, with many more possible explanations.
As I believe Tenesema stated, one of the elements to anesethesia seems to be the prevention of such disasters. Still, Bad Things happen. And when they do, it can be quite stressful.
Alas, these are just personal observations. I'll freely admit that I'm unqualified to say any more on stress levels. Would love to hear opinions from any residents out there.