One thing to look out for as far as design is whether the floor has texture, or "embossing". Also bevels, the grooves between planks, help create a more realistic look. Finally, a low sheen level hides the plastic look of vinyl, and can give it a more natural feel.
Other details are width and length of the planks, and of course the thickness of the wear-layer. A thicker wear-layer will result in the floor lasting longer, which will keep it looking its best.
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