Back in the days when Firewire was popular (sigh), people were importing video over Firewire from their miniDV camcorders. This video was a fixed rate of about 3.6 megs per second, so an hour of DV video takes up 13 gigs of hard drive space. You can imagine a couple hours of vacation footage, plus the scratch space needed to edit, took up a pile of space.
NOW, with High Definition video, it takes up even MORE space, especially if you're using iMovie or Final Cut Express and converting things into the Apple Intermediate Codec before you edit.
Plus digital cameras are constantly getting more megapixels, so photos are getting larger... what used to be about 400KB per image is now 4-6 megs per image, easily.
Higher-resolution, high-def video downloads from iTunes, DRM-free and less compressed iTunes music, it doesn't sound like much individually but it all adds up very quickly!
My 12" PowerBook had a 40 gig hard drive which I kept filling. This MacBook Pro has a 120 gig hard drive which I thought would be plenty... now it's pretty full and I'm looking at replacing it with a 300+ gig HD...