Well, I think there's a fine line between "hoarding" and "collecting". I like to collect different diapers from different companies but usually use all but one and save just the one. Eventually though, I use up the one and move on as I'm not much for saving anything for a long period of time either. Also, many may have a "stash" of diapers but it could be for reasons like a good deal, delivery methods, whatever. They wouldn't really be hoarders in the negative sense either. I mean, we're all hoarders to an extent.
Hoarder is defined by allwords.com: One who hoards; one who accumulates, collects, and stores.
Well, you could say if you have more than one stamp you're hoarding, or more then a couple cans of peas a hoarder, or someone who has any sort of collection a hoarder.
The hoarding the people have on this show is a different type and is an actual medical condition.
Pathological Hoarding defined by medterms.com:
Pathological hoarding: Excessive hoarding of material goods, a condition that affects up to 40% of people with obsessive compulsive disorder (OCD). Excessive hoarders, who fill their houses with accumulations of junk, usually newspapers, bags of old clothing and lists, tend to experience more anxiety, depression and social disability than OCD patients with other symptoms. Hoarders are also less likely to seek help.
Pathological hoarders have decreased activity in the anterior cingulate, a brain structure involved in decision making and problem solving, compared with people with other OCD symptoms. The hoarders also show less activation than the healthy subjects in the posterior cingulate, an area involved in spatial orientation, memory and emotion. Hoarding may therefore have a distinctive basis in brain activity.
Also check out Wikipedia: Compulsive Hoarding
At this site you'll also discover book hoarding, animal hoarding, and digital hoarding. Most cases involve some sort of OCD and the hoarding is somewhat considered a disease when it affects there health, living conditions, mobility and the items are useless or the items are in an unmanagable number.
So hoarding really all depends on how you look at it. To me, my diapers are a collection as I use and discard them and the unused ones I save, don't affect anyone or anything around me.