With enough experience, you start to see the same problems cropping up.  Although this list and the descriptions may change after further reflection, here are some problem personality types that seem to have difficulty getting to the end of graduate school:

The perfectionists, who think that everything has to be black and white and perfect before they graduate.  Nope.  These are the absolute worst to deal with, they always have a reason to not graduate.

The idealists, who think that their work has to be ground-breaking before they graduate.  Nope.  Not everyone gets to be that lucky.  If you are, awesome; if not, save it for post-graduate work.

The procrastinators, who seem to think that the work is going to be too hard and that graduation will happen simply by “hanging in there” and keeping warm and breathing. Nope.  You have to get off your duff and start performing the tasks.

The starry-eyed students, who are now thoroughly disillusioned with academia and/or their mentors and advisors and academia in general, and are ready to give up after four or more years of work.  Nope.  It is your life, of course, but you will probably have more options if you finish the degree out.