The setter is the one player who will contact the ball every single time it is on your side of the court. Thus, the setter must be able to get to the ball and set it well. The most important attribute for a hitter is kills. Sure, it impresses everyone if you can hit the ball straight down so hard that it hits the ceiling, but a player like that is often easy to block. Getting kills is what is important.
Height does play a crucial role, however.
Use your strengths, hide your weaknesses. This may seem obvious, but many teams don't do it. Yes, you need to put together the best team you can with the personnel that you have. If you have a player that is an awful back row defender, then sub him out for a better defender when he is in the back row. This rule also implies that you should use whatever works.