Knick knacks that cover ever surface so that there isn't even room to put down a mug of tea so you have to hold it until it's drunk.
Overhead lights that are so bright they hurt your eyes - I think some folk have stocked up on those 100W bulbs you can't buy in shops anymore.
Aggressive dogs - especially the one that shredded my newly knitted hat made from hand spun, hand dyed yarn. It's owners didn't bat an eyelid and made no attempt to retrieve the hat from the dog in case it bit them.
Wooden floors with no rugs. They are just so stark and bare (especially with previously mentioned bright lights)
No towels in the bathroom so you have to dry your hands on your clothes. I assume those people never wash their hands after using the loo.
People who never put the heating/fire on regardless of how cold it is (MIL I'm looking at you, she tucks a blanket around knees but the rest of us sit and freeze).
People who have toilets that are "temperamental" so flushing is a knack. Well me and other visitors don't have the knack and it's just embarrassing to have to flush it 3 times before whatever's down there is flushed away. Just get it fixed ffs.
And definitely houses with no books - I'm very judgmental about your inner lives
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