ClearType which is probably the best reason for upgrading if you have an LCD screen.

For me the boot/hibernate/shutdown times are the killer feature. My aging NEC laptop now takes longer to go through it's BIOS startup sequence that it takes XP to resume from a hibernation. It means I can "boot" (i.e. resume from hibernation) in a minute an a half rather than the five minutes boot time under Win2k.

I played with a friend's Sony SRX81 that boots in about 30 seconds with XP, wonderful.