First, there are two questions in the OP, good and great. As someone else here mentioned there is a huge difference. There are a great number of good players breaking even to small winner in casinos around the country these days, the very good players are making money, the great players are few and far between.
For the very good to great debate... Marsha and dr strange nailed it. Natural ability teamed with tons of hard work and dedication to learning and adapting to ever changing games. I know the 1-2 game in your casino hasn't changed in 5 years, but the big games in Vegas, LA, San Jose, Miami, and Cincinnati (yes, Cincinnati, Ohio) are constantly evolving. The reg v reg battles that the very good to great players have to fight every day are incredibly complex with tons of history and overt aggression. Not to mention that the players have to worry about the ever-increasing skill levels of their marks. It's a tough world out there for the want-to-be great players.
I'm far from claiming to be great, so don't misunderstand this - I learn a ton every time I go out to Vegas. It is fascinating to learn new techniques, range balancing ideas, bet sizing, and position raising concepts.