I think it becomes a problem when it's overflowing everywhere and you can't store it neatly. When you have little trails to walk through between the mountains of stash, you just MIGHT be a hoarder.
We need Jeff Foxworthy to write us some funny "You might be a horder" lines!
BTW, hoarders don't usually worry that they are hoarding, so just having considered it surely you can't be a hoarder, LOL.