Will it fluff?Some will some won't. Some will more than others, some will for a while and then stop.
Will it flatten? Some will some won't. Sometimes that doesn't matter. They can still look good.
Will it shade (watermarking)? This usually occurs with very plain, plush pile rugs. It is where the rug can change colour in certain parts and not others. You often see it in plain carpets in large commercial areas. In some rugs it is unavoidable.
Will the pile pull out? Sometimes they do. Slippery threads, like rayon, in modern shaggy rugs have some fibres that will pull out. This is normal and unavoidable in many cases.
Will the dye transfer? In 99% of moderns rugs, the dyes are extremely 'fast'. However, in some Persian rugs, older vegetable dyes are used and colour fastness cannot be guaranteed.