I have had a server always on at home since the days of Fidonet BBSs (I used to run one), and so I am used to have a machine always on in my home that I can rely on.
Currently, my home server hosts few websites, my data, my gf's data, my picture and music collection, and more stuff.
Since I don't keep (yet) a video collection in it, 2TB seem to be enough so far, even though my recent interest in photography is (not so) slowly pushing me to considering an expansion.

In any case, I have a RAID-1 storage system including system and data volumes.

In addition to that, I have three more "on-line" 2TB disks that I use for automatic daily, weekly, monthly backups, which, over time, I found are more than enough to address any deleted-by-mistake issue.

And finally, I have two "off line" USB disks I use for manual backup.

This has worked very well so far. I keep considering an on-line backup but I never got myself to getting the whole system setup, even though I like Crashplan. My bad.

Now, besides the obvious advantage of eliminating the down time in case of a system disk failure and consequent restore from backup (I never gave up the mentality of having the home server up and running at all times, even though in these days that is not as important to me as it used to be), the RAID-1 allows me to expand my drives quickly and with just 2 reboots (as I don't have hot plug storage in my case). If it wasn't for that, I would probably reconsider a simpler model.
On the other hand, my home server also serves a purpose of testing, experimenting, and fun, so some "unnecessary" redundancy is welcome anyway.

In fact, as Mark here recommends a good 3TB disk, I'll probably get two. Also because people in here convinced me that RAW pictures are obviously a good thing to keep.