I'm a little jealous! Why can't I develop one of these really interesting obsessions?
Anyway, if I were to list them all, I'd most likely crash the server. Over the past few decades, there have been literally *thousands*. There are a few recurring ones, however . . .
I'm obsessed with proofreading -- to the point where I don't really even try to read anymore. I can't read books, or chapters, or even sentences. (Not pleasurably, anyway.) I read letters and punctuation marks. I'm also generally obsessed with ordering -- numbers, dates, events. What came first, how does this event fit in with that one? Trying to make sense of (usually pretty meaningless) sequences of events. At the moment, I'm trying to figure out the translation history of an obscure Bible edition starting in the '40s and finishing in the '60s. What individual books were translated and published in what order? This morning I was trying to track Ted Bundy's travels around the West in the mid-'70s. Where was he and when? (Creepy, I know.) Generally, I love to investigate.
I also love to memorize: everything from the codes on vacuum tubes to prayers in Latin. The typical stuff, too, like pi to however many decimal places.
What I've figured out is that, so long as it really doesn't mean anything or have any significance, it's fair game to obsess over. It's the substantive stuff that scares me. Ask me to really commit emotionally to something -- i.e., develop a real hobby that I care about -- and I'll just stare you blank in the face. Weird and sad, I know.