My in-laws both have hoarding tendencies. Both of them lost a parent at an early age and grew up without much money. So they have a hard time throwing away anything, and if it's a "good deal" they have to stock up. They have more food in their home for the 2 of them, then I do for my family of 4, and I don't think they ever really cook.
Things that drive us up the wall:
Knowing that they have a lot of debt, yet every time we visit they've bought new toys for the grandkids, that had to stay at Grandma and Grandpa's house. One time when we were visiting FIL gave my son a little toy train to take home. He said they had gave it to my son at Christmas, but we had left it at their house. When MIL heard, she had to come look at the toy to make sure we weren't taking one of hers. We offered to let her keep it, because we knew that our son wasn't that interested in it. She insisted we take it, but then later said how she would have to order another one on-line because they only had 2 of that particular train car. These were not collectibles or anything, just little trains that the kids played with on the train table. I don't know why they'd even need 3 of them.
Getting them a nice gift they specifically asked for, only to discover that they've never even taken it out of the box. Like my MIL asked for some nice drinking glasses. We bought the ones she wanted, and they sit there unused while they drink out of plastic glasses from fast food restaurants. My FIL does the opposite, he will use it, but doesn't take care of it at all, so it's trashed within a couple of months. I know once you give a gift, it's out of your control, but it's so frustrating!
Listening to them complain about the others stuff.
Not being able to show them or talk about anything we have bought recently, because then MIL is convinced she needs one too.
It bugs DH and I, but we try to stay out of it...at least until they ask us for money. It drives BIL nuts and he's always trying to do something about it. Last summer, when my ILs went on vacation, he cleaned out their garage (with their permission) so they could park in it again. By the time winter arrived the garage was already too full to park in once more.