Andrew Chen has a great post about a common thread for me, the misuse of user research:

Ultimately, quantitative metrics are just another piece of data that can be used to guide decision-making for product design - you have to combine this with all the other bits of information to get it right.

Just like there’s no single risk-assessment equation in finance, there’s no single driving factor toward successful interface design. While user testing and research is an important factor in any significant design project, conclusions that are principally based on user research are incomplete and underdeveloped.