First of all, you should start with colour. This is probably the most important factor when it comes to ensuring a rug looks good in a given space because the colour scheme has to be maintained. Only shortlist rugs with the right shades and hues in their patterns.
Pattern is another issue. Pattern often determines the overall style of a rug so you need to decide whether you want a patterned rug or a plain rug; whether you want a simple rug or an intricate rug.
Then you can move onto the question of texture. Lots of rugs exist out there: shaggy rugs, modern rugs, natural fibre rugs and so on. The all have different textures so read the descriptions carefully depending on how you want your rugs to contribute to the environment in a space.