ANYTHING most buyers would not want; most would need to spend the money and time to eliminate this factor.
It can be as simple as an unusual paint color inside or out.... or maybe weird flooring or carpet color. A potential buyer may think "I'll have to repaint... recarpet" and thus lower what they are willing to pay since they'll need to do that work.
Thus, anything unusual that a potential buyer would think needs to be changed.
Not necessarily even anything unusual. If your buyer just doesn't like the color of the carpet or the paint they'll be thinking about the cost of replacing it even if they aren't thinking they'll want the work done before they even move in.
By the time you try and figure that something light looks airy but shows dirt, something dark doesn't show dirt but can look closed in, blues can make some people feel cold, oranges and reds can be too vibrant for some people, there's really no knowing what will appeal to a future buyer and what will put them off.
It's interesting to see the concept of something that "needs" to be changed. If you don't care for the color of the carpet you don't need to change it, you'd choose to change it. Even if the walls are a lurid shade that only works on insanely expensive Italian supercars nothing is forcing anyone to repaint.
