Stick-on wallpaper borders.
Wallpaper with enormous gawdy flowers on, especially red on black.
A UPVC front door. I accept the sad inevitability of buying a house with UPVC windows but the doors are downright ugly and cheap looking.
Fitted bedrooms where the units go all around the room, up to the bed and over it. It looks like a caravan.
Storage heaters
Breakfast bars that don't function properly as comfortable places to sit and eat. Especially the ones that face a wall, or don't have enough overhang on the tabletop to be able to tuck the stool and your legs under properly.
About 4 en suite shower rooms, all the size of broom cupboards, with no windows and no space for a decent sized shower or any storage. Just give us two decent sized bathrooms instead, one of which may or may not be en suite. Much better.
Really narrow warren-like corridors
Really steep angled stairs
low ceilings
not enough natural light
An awkwardly shaped, poorly laid out kitchen that doesn't flow.
Sitting rooms that are too narrow
A TV on the wall, placed way too high, with a strip of plastic conduit stuck vertically to the wall to 'hide' the cable. It doesn't.
A DFS or DSCS sofa, especially if it's that cheap, rock hard, shiny, unforgiving leather that doesn't yield when you sit on it, so you slide straight off.