Rating speed on a draped rein - your horse should go fast, medium, and slow on a comfortably draped rein, when you ask him to.
Cadence - the horse should have good rhythm and keep a constant "beat", no speeding up/slowing down all the time.
Body isolation - the horse should be able to isolate the hip, ribcage, and shoulder by using your leg.
Beginning collection - the horse should move into the bridle with good impulsion, driving from the hindquarters. Then you encourage the horse to lift the shoulder, lifting the topline, and promoting good collection with a slow leg.
Remember, a good western pleasure horse is not a "lazy" horse, they are using a lot of their body with a slowed cadence - not short, choppy "quick" legs. :)