A topic that I've found a renewed interest in recently is differences in personalities. It started with talking to someone recently about the Myers-Briggs Personality Type Indicator, which basically puts people into one of 16 personality categories. You're supposed to take the "official" test, which you can learn how to do here. There are also a few fake online ones floating around out there. (For what it's worth, I've taken both with the same result.) Basically, there are 16 Myers-Briggs types, because there are two "preferences" for each of the four categories they measure personality with. The four categories are favorite world (introversion or extraversion), how you get information (sensing or intuiting), how you make decisions (thinking or feeling), and how you deal with the outside world (judging or perceiving). For example, your personality could be ENTP, for extraversion, intuiting, thinking, and perceiving. (My personality is, to no surprise to anyone who knows me, INTJ.) I sort of like this kind of qualitative measure of personality, because it's quite non-judgmental, and forgives us for being different (for example, I can't feel guilty about not wanting to socialize for hours at parties; now I can think that it's not a character flaw, being introverted is hard-wired into my personality). I also think that we are pretty much born with our personality, and it changes very little. (Which might explain why I was such a terror as a child, sorry mom and dad.) There are a couple websites where you can read about the different personality types, and from the people I know, the descriptions seem pretty accurate.
The second reason I was thinking about personality differences is because I am doing a psychiatry rotation, and I feel like there's often a very fuzzy line between personality and pathology; i.e. what some clinicians want label psychiatric disorders or personality disorders I sometimes have a hard time viewing as little more than differences in personality. I mean, where does the line between high-energy and hypomanic lie? There are, of course, real, quantifiable criteria for diagnosing, say, depression or generalized anxiety disorder, but people who fall on one side or the other of the diagnosis aren't radically different. I think we're all lying along a spectrum somewhere, and the idea of where personality differences end and pathology begins is an interesting one.
I highly recommend looking into the Myers-Briggs stuff, if you never have. It's very interesting, and might tell you something about yourself or how you interact with others that you never knew before. When I read about my personality for the first time, it definitely taught me something (and, alas, not all of the information was good ;) ).