Justin Esarey has a blog post at The Political Methodologist with suggestions about what prospective Ph.D. students should be thinking about. It has very good, practical advice, though you have to sort through a lot of bold, italics, and CAPS. However, one bit puzzled me, when he got to a list of things to think about:
Your personal love for or “calling” to the field in question. As many people have pointed out recently, “do what you love” isn’t a sound plan for graduate school. In short, not everything that you love will love you back.
I understand the sentiment. Many subjects that people love are not well connected to employment. Got it. But. Your love for the topic should actually be the very first point about why you are getting into this lengthy, difficult, and time consuming enterprise in the first place. If you do not love this topic, then why are you even starting? If you start backwards and look first only at completion rates, etc. and you don't pay enough attention to whether you love it, you may be dooming yourself to many years of unhappiness because this is going to consume all your time.
So my advice is to start with what you love, then do a lot of legwork to make sure what you love has a reasonable future. And if you don't love it, then please don't start.