itsbob
I bowl overhand
It's not really the low f-stop, it is the consistant low f-stop throughout the lenses range. Its easy to build a lens that has a low f-stop at its shortest but quickly loses light the longer the focal length becomes. There are also better coatings on more expensive lenses that do things like get rid of purple fringing. The clearness of the lens at the edges also is a big one, many kit lenses are soft around their corners but the good ones, like Canon L's, are tack sharp corner to corner. The cheap lenses are often decentered, being sharp on one side of the lens but soft on a very big portion of the lens on the other side.
That's what I said!!