I have been fascinated and often repulsed by the programs "Hoarders" and "Hoarding: Buried Alive." These two programs have brought this condition out of the closet and laid bare for all to see.
What is hoarding? Hoarding: (1) the acquisition of, and failure to discard, a large number of possessions that appear to be of useless or of limited value; (2) living spaces sufficiently cluttered so as to preclude activities for which those spaces were designed; and (3) significant distress or impairment in functioning caused by the hoarding.

I am willing to bet that almost all of us have known people that qualify as a hoarder or are on their way to becoming one. Before it had a clinical definition, those people were probably like "Granny N" or "Mr. & Mrs. R" who were known for having a messy house and piles on top of piles. In our little community, Mr. & Mrs. R have been in trouble with the town for extreme clutter. They are now in their third house. The first two they still own but can't live in or sell because they are packed to the rafters with stuff. I can remember a granny of a friend of mine whose house was so cluttered that it was difficult to move around. When we were kids we would go to her house and I was always amazed by the number of boxes that could fit in a tiny room. Then there was my husbands aunt and uncle, now deceased, whose house was discovered to be so filthy and disgusting that the city tore down the house and just used heavy machinery to clean it off the lot. It was an old shack of the house anyway. When they would feed their cats (unknown how many that really was) they would put the canned cat food on a paper plate and drop it on the floor, never to be picked up again!
I tell my husband that if he isn't careful, he will slide into that pit of hoarding. He can be a bit of a pack-rat now. I have to work hard to keep his
crap stuff out of the living spaces of the house. The two car garage hasn't held a car in probably 8+ years. The kids will say that we have two yards. Dads yard, on the south and west sides of the house and my yard on the east and north. My yard has the gardens and is kept clutter free. You might see a garden tool or a wheel barrow sitting from time to time, but those are when work is in process. Dads yard is a loose arrangement of farm implements, tractors, empty tubs, the plastic liner from the truck with more
crap stuff in it. Immediately to the west of the house isn't so bad really. The swing set, sandbox and patio-esque area is really normal backyard play and socialize space. Just go beyond the electric fence though, and you cross back into pack-rat land. There is a wagon of junk in the barn, some piled off to the side in the barn and more junk behind the barn. All
crap stuff that can come in handy someday, of course.
Just the other day I had him helping sort out some stuff in the back room entryway called the "mudroom." It is just a 5' x 10' area that is for setting things down and storing the dog food, winter outerwear, boots, etc.
As he was picking up the excess, he tried to hand me a small pile of reflective vests that he has collected this summer. He wanted me to put them in his closet with his other box of these. I told him that if he put them in there or expected me to find a place for them, the dumpster is where I would put them. I think he ended up putting them up in the rafters of the garage.
Separating this area from the game room is a couple of shelving units. I have put backer board on the back side of these to make a "wall" for the game room. The main problem we were addressing was the fact that the mudroom had started sliding over into the game room area. With winter and holiday get-togethers coming, I needed this room cleaned up for people to play Wii and sit and eat if they want.
Anyway...I digress...
So when does collecting and pack-ratting slide into hoarding? I think it depends on the items involved and the intention behind saving them. I have things I save and stock-up on. Anyone who thinks about preparing for emergencies or hard times does that. Any salvaging things that can be reused makes sense sometimes. However, piles of trash that most people would get rid of, not having more than pathways through the house, not having the use of actual living space, and compulsive shopping just for the 'high' of it are probably all indications of turning that corner.