I suspect that it takes an innate talent for story-telling to interpret the computer-generated behaviour of the characters into a meaningful plotline -- a lot of the Let's Plays I've seen have been challenges of various kinds, where there's an external constraint of some kind on what you're allowed to do.
(Apart from that, the basic activity does seem to consist of 'make Sims get married and have babies' for the female players and 'kill off Sims in bizarre ways' for the males ;-p)