An important element to consider when decorating small spaces is to only use items you love. In small areas there won't be room for items that don't make you happy.
A room or area full of mismatched pieces won't look as nice as it could. Even if the pieces are expensive, well made & comfortable, if they don't match, it'll show.
Not in regards to the actual seats, but the overall comfort of the room. For example, a huge abstract painting may dominate a room or a mountain of throw pillows may be uncomfortable.
It should go without saying, but the décor for a 10-year-old girl most likely won't appeal to a 16-year-old boy or a 50-year-old man. In group areas, gender & age neutral work well.