Location, exposure, neighbours, noise on first sift - but nothing you can do about those.
I care about good bones, so I look for roof, windows, wiring, plumbing, flooring, all high-quality and well-maintained. Newly done work is great if I like it, but off-putting otherwise, as extra expense to put right.
Awkward layout, lack of natural light, poor ventilation and lingering smells are a no.
Ancient kitchen is a plus so I can justify ripping it out (I prefer freestanding). Bathroom & toilet similar, but less so. I don't care at all about your decor or furnishings or clutter, or how you've arranged the rooms: I just mentally subtract all that anyway. But I do want to see the entire space available, including inside all built-in cupboards - so not a good stash space for clutter during viewings!
I don't want to pay for the latest interior decor trends.
I'd rather have a huge ramshackle garden than an overly-pruned chemical-soaked one where I have to rip out patios & garden features and spend years restoring the soil.