On what factors is a dissertation approved
Although the exact process varies from university to university, the general form of the process in the United States is typically as follows:


The student needs to do work that makes a novel contribution to the sum of all human knowledge.  This is the universal part of the process, and the key thing that a Ph.D. really certifies that you are capable of.
The student, with guidance from the advisor writes up this work in a thesis document.  In some areas this is not-yet-published work, in others it is a summation of things already published.  In either case, it should be a complete and thorough presentation of the ideas and work.
Eventually the advisor and student are both satisfied, and the advisor judges that the student's work will pass external review.
External review is obtained from some combination of oral or public defense and input from other committee members.  Sometimes the defense is a serious part of the process, sometimes it is not; in either case, an advisor should never allow a student to stand for a defense unless they will pass.
The student satisfies any additional requirements imposed through the external review.  Sometimes this is just correcting typos, sometimes this is a lengthy period of additional research.  If it is the latter, then the advisor has generally screwed up.
Submit the final document for archive, and graduate.


Note that this process assumes some flexibility in the length of a Ph.D., which is typically the case in the US.  I know that in some other places, such as many institutions in Europe, there is often a shorter and fixed schedule.  I'm not sure how one deals with not being ready on time in that case...