Fill it with non-family friendly entertainment like we've always been doing, I suppose?
Actually, HDD size isn't my main concern, but lifetime. When 2GB-80GB drives died back in the day, it was no big deal, their small size meant we were pretty much forced to burn backups every other week or so. Nowadays with 320GB and 500GB machines, it's too easy to procrastinate: "there's still a lot of space left".
Now imagine if you had everything all in one huge drive, and on one fine day it decides to go south. Not looking forward to that. I'd rather have a RAID 1 array rather than one huge basket with all my eggs in it.