Really only very little of that comes into play when choosing a new lens. You should buy a lens that will serve it's purpose for what you plan on using it for.

The number one rule of thumb when it comes to lens' is don't skimp on your glass. Buy the best you can afford, and keep in mind if it is to be used on a cropped sensor body or a full frame body (for those of you who are digital shooters). If you plan on upgrading from a cropped sensor body to a full frame body don't buy a crop sensor lens' as you wont be able to take full advantage of that new full body sensor on your new camera.