For many of us who are just starting to figure out what we want out of graduate school, what we want to do with our lives, etc... this is a fairly helpful article to get you thinking about the difficult topic.
http://www.gradschools.com/article-detail/what-to-study-96For those of us who are concerned about what it takes to be a successful PhD student, here are a pair of articles courtesty of Matt Might that I personally love and I think are very illuminating.
PhD in pictures:
http://matt.might.net/articles/phd-school-in-pictures/3 Important Qualities:
http://matt.might.net/articles/successful-phd-students/