in very close order:
Head Trauma;
Abdominal internal bleed
Broken femur / hip
Arterial bleed

The arterial bleed can be accurately accessed and is fairly field treatable if you are on scene when it happens. (eg Aaron Ralston and the tourniquet) You can usually control arterial bleeds with direct pressure - indirect pressure - tourniquet approach, with the notable exception of the carotid artery.

Head Trauma may result in spinal injuries that are not noticed untill a while later when you suddenly fall down and cannot move or breath etc. Also Head trauma will lead to concusion and (hopefully temporary) stupidity which will lead to other bad things.

Abdominal internal bleed may not even be noticeable until so much blood has been lost into the abdomen that there is unrecoverable shock setting in.

Broken femur / hip is not only severly painful and debilitating but often leads to internal arterial bleeding into the groin - see abdominal internal bleed above.

For field situations where definitive care may be more than 24 hours away I'd prefer my patients dead or walking <img src="images/graemlins/wink.gif" alt="" />