The main thrust of arguments can be summed up as:
Macs:
Look better
Work better
Play better
Make you a better (smarter, cooler, sexier) person
PCs:
Run more stuff
Can be tinkered with by anyone who wants - wisely or not
Had two-button mice before Macs did.
It all seems fairly arb, but generates a great deal of passion on both sides. I suppose in these post-nationalist days (well, everywhere but the US) people need some identity flag to rally behind, and religion doesn't do it for too many people any more.