"To grab people's attention, you should argue that something we think of as bad is good, or vice versa; that some apparently individual phenomenon is really collective; that several seemingly disparate things are actually part of the same thing."
From this Oliver Burkeman column on interestingness v truth. This definitely holds for the majority of arts PhD theses, in my experience. My own could admittedly be summarised as follows: "Ooh look, here's something that's interesting and allows
apparently disparate things to be linked, yet has been overlooked even
in two of the most pored-over novels of the twentieth century".
(Thanks to Cat for the link.)