The servo mechanism subsystems are the motor, the gear train, the potentiometer, the electronics, and the output shaft bearing. The electronics are pretty much all the same and so not an issue. In the motor department however you can get smaller and larger motors which effect the overall size of the servo. The gears also vary from servo to servo. Inexpensive servos have plastic gears that will wear out after less than 100 hours of use.
More expensive servos have metal gears which are much more durable.
The potentiometer is the feedback device and often the first thing to fail in my servos. I've found I can extend the life a wee bit of my pots by using some judicious application of silicone sealant around the edge. You can do this with a syringe if your careful. Be sure and not to get it on the gears though as it will cause them to bind. The last subsystem is the output shaft bearing.