By riding more horses I have learned:
-to respect my OWN horse more because even though he is not always perfect...he is much more willing and trusting than some of the others.
-how to evaluate my rides with more understanding and description than previously.
-how to become a better rider. Each horse had their own ups and downs, and I had to adapt to each and every one of them. One horse like to put his head down at the walk and trot.. so I had to sit up and sit deep (something I am not always good at). One horse had a cold back and was a hunter that didn't like to accept the bit. I had to learn to stay off of his back and use light contact because of his sensitivity, but also work on keeping him supple and relaxed so that he would learn to accept the bit. Others required more leg than mine, and some rushed into the canter.
So, if you ever have the chance to ride another person's horse (but not a crazy one!) then I guarantee you will learn a lot about yourself, your own horse, and the way that other horses are ridden.