Extra dust bunnies is all. Just make sure to clean it once in awhile so your cellunoid intakes dont get completly clogged, I've seen motherboard actually spark and catch on fire when that happens ed:
i popped open my moms 1997, 750mhz beast the other day, there was soot up the side in one spot where it had apparently caught fire.... strangely it still works fine... or as fine as that shitty of a computer can
oh ya and enough dust to make a life size dog rock: i just stuck the vacuum in the mofo