When I read the title, the first thing that really came to mind was patience. I don't think that patience is the most important element in becoming a good or great player, but I think it is often overlooked.
Here's my take on patience:
I have always felt that pool/billiards has always been a sport/game where the person that wins the match at hand isn't ALWAYS going to have been the 'best' or 'most skillfull' player. A higher A players game will always be vulnerable to a B player that's getting the rolls. In other words, the luck factor. I believe it's huge, and I think it takes a really high level of skill to be able to overcome the luck factor in pool.
I believe that luck is a factor in any sport, however I still believe that in most sports, eventually the guy that comes out on top is the best player for the day.
I've won and lost many matches and tournaments and I can't think of a single instance where some form of luck didn't play a large factor in the results of some of my matches. I've rolled over players that I should have never beaten simply because everytime I missed they were hooked, or everytime they missed, I was wide open. And I've lost to much lesser players due to the same circumstances.
So this brings me back to patience. When you've come to the conclusion that you want to be a good or a great player, you have to be prepared to accept that fact that many of your results in the win/loss statistics will have been affected by luck in some form. And you'll need the patience to accept that luck will be a part of your game, good or bad, until you can bring your game up to such a level that luck, good or bad, will hardly matter.
I truly believe that only at the very highest level of pool, is where you'll find results where the most skillfull win every time. (SVB, Appleton, Archer, and so on)
dave