Conservatory. Blocks out light, conducts all the cold in winter, all the heat in summer. Full of bugs I'd have to remove it, big expense, one less room.
No off-street parking.
If attached to another property, no houses that don't have one large bedroom and one general room not against the neighbour's wall.
Main bedrooms all sharing walls, especially stud walls.
'Internal' rooms i.e. windowless rooms, or too many internal walls removed for 'open plan'.
Older than 1950s, or made with non-standard materials, or having non-standard services.
Wood-burner if it would be a pain to remove.
'En-suite' bedrooms.
Houses where future close neighbours' extensions could significantly block my light.