Many trainers gain a base knowledge of how to train clients. This is great and dandy, but your trainer should continually learn and grow as a trainer.
Training Everyone the Same
People are different, and so should their training programs be. It's that simple.
Foregoing the Fundamentals
A great trainer reinforces fundamentals on a continual basis. Bad trainers get jerked every which way by fads, flashy equipment, and the current trends in training.