An inability to break big mushy problems into a series of small, doable, concrete sub-problems. This, of course, is one of the things students go to graduate school to try to improve on. Everything is possible when talking about big ill-defined problems. But getting some of them to focus, commit, and complete is harder. Lack of practical skills is sometimes a problem.
Students have different motivations.
Some really want to avoid the pressure of being a professor with its necessity to face all those classes and fight for tenure. Some want to entrepreneur. Then there is the factor of how many jobs are available and for what sorts of people at the time students are making this decision. You can more or less look at the way they walk and talk. We have been getting incredibly good students at PARC for the summer recently.