PVC windows. Those plastic conservatories that people loved to jam onto houses in the 80's. Concrete tiles on the roof. Obviously all the other stuff that people have mentioned - cigarette smoke, dirty house. Once I was shown a house with tenants in situ, and got a eyeful of one of the tenants' prick and arse. Didn't get beyond the front hall before backing right out.
TBH, we've spent the last 15 years getting our house sorted, and it was already well cared for and had good bones. Some crap plumbing, a fireplace that was basically a big hole in the chimney breast, and we've changed all three bathrooms, ripped up carpets and sanded and sealed floors. Added decent insulation and improved lighting and landscaped the gardens, front and back. Built in storage and decorated. New boiler, better rads. And that's before we've even touched the kitchen, which is the next big thing.
So there's a big bit of me which never wants to move again, and certainly not to an old house, however pretty, that we have to sort out properly to our own spec and preferences. I'll be looking for contemporary, with everything to high spec, maybe trade some floor and garden space for high-end design. Neff house, that sort of thing! If anyone is selling...