Three qualities of a successful Ph.D student:
1. Preservance- To escape with a Ph.D., you must meaningfully extend the boundary of human knowledge. More exactly, you must convince a panel of experts guarding the boundary that you have done so. You can take classes and read papers to figure out where the boundary lies.
2. Tenacity- To get a tenure-track professorship after Ph.D. school, you need an additional quality: tenacity. Since there are few tenure-track faculty positions available, there is a fierce (yet civil) competition to get them.
3. Cogency- Finally, a good Ph.D. student must have the ability to clearly and forcefully articulate their ideas--in person and in writing. Science is as much an act of persuasion as it is an act of discovery. Once you've made a discovery, you have to persuade experts that you've made a legitimate, meaningful contribution. This is harder to do than it seems. Simply showing experts "the data" isn't going to work. (Yes, in a perfect world, this would be sufficient.)