Sticktime
I think one of the most important things is the amount of time you spend physically in control of the machine. A student thats been watching a instructor going through the lessons, will not be as well prepared to go solo as a student thats been in control throughout his lessons.
Of course this depends on all those factors previously mentioned. How well a student prepares for lessons before climbing into the cockpit will determine how much time he/she wastes, and this will have a huge impact on solo time.