Money to pay for proper care is important, but to me it's not what makes a good owner. As one of the previous posters said, it doesn't take much to call out a vet or a farrier.
A great horse owner is one that starts out with an open mind, a strong driving desire to improve, and immeasurable patience. I don't really think that someone really experienced necessarily has to be the greatest owner (though they are really good), because nobody just starts out being totally awesome and knowledgable. Someone with no ego, and is man enough to set their pride aside and admit that sometimes yes, they do need help. It's also someone who realizes that no matter how great you think you are, there is no limit to the amount of horsemanship you can learn. And of course, someone who loves the animals they're working with.