I enjoy studying common problems in human behavior. They're interesting because:
Some of them can be downright spooky, revealing really interesting details about perception.
- they occur to the best of us
- though they're typically small problems, they can have very measurable effects when accumulated across larger populations (e.g. stock market trading)
- as with black box testing, failures teach you far more about how something's internals work than the successes
- studying them has practical side effects: you become better equipped to avoid them, and (if that's your game) better equipped to exploit them in others