I think the point of this idea is not to really only have 100 items, but to challenge yourself to look at your space. Dave wrote, "stuff can be good when it serves a purpose greater than possession alone."
Just look at this sentence and the word "possession". The issue is that most of us allow our "possessions" to "posses" us.
We keep a lot of things because they were gifts, or because they remind us of a special time. If you let that item go, will you forget that special time?
If you do, then the time must have not been too special.

Years ago people easily lived with less than 100 items. Of course I'm not saying lets live like cavemen, but the fact that we can't is what holds us back from growing to our highest potential.

DCer said his kids each have a box of 64 crayons. Why? Can't they share one box?
If they aren't able to share a box of crayons how do you think they'll be sharing, not just material things, but thoughts, ideas and feelings in class, home or even when they grow up in the work place?

As for me I don't limit myself to a number, but try to go by if something comes in, something else goes out. Or every few weeks/months I have a closet clean-out (I did this yesterday). It's easy to clean out my closet because I don't have so many clothes or shoes. It's also easier and more pleasurable to get dressed because I don't have to go through tons of clothes to choose the one I want to wear.
And I do agree, if you haven't touched it in 12 months, let it go.