This video review shows things to look for in lens quality, such as looking specifically for chromatic aberration and what that looks like. In this example the aberration shows as purple fringing. In some lenses it can be cyan and/or magenta. In addition, he shows what barrel distortion looks like. Pincushion is exactly the opposite, and he show what an image with soft corners looks like.
The reviewer also points out some difficulties in using this particular lens due to a somewhat less than smooth glide during zoom and why having variable aperture can be a problem.
For anyone who doesn't have a lot of experience with lenses, I think this video could be an eye opener.
[youtube]http://www.youtube.com/watch?v=73thfbZIDuc[/youtube]