Speed-torque curves
Almost everything you need to know about a motor is contained in the speedtorque curve. If you know where to look, you can find or get a feel for such critical information as peak operating levels, stall conditions, starting torque, and dynamic response. Speed-torque curves can also help you select the best motor technology, whether it's ac or dc, stepper or servo, moving coil or switched reluctance.
questions & answers
Why would a "properly sized" motor stall in actual use?
It's probably overheating. Use a derating curve to select a different motor, and see if it doesn't perform better.
Is it better to use a servo or stepper?
The best motor is the one that does the job. However, if the application falls within a step motor's safe operating range, you're likely to save money with the stepper.
What would cause a motor to fall short of the acceleration predicted by its speed-torque curve?
Friction, mainly. To overcome it, make sure you design in some torque margin when selecting a motor.