There's a few things that indicate the quality of a lens.
- Build Quality: The first, especially if you smack your equipment around. Smooth auto focusing, well dampened zoom rings, button layout etc fall into this category.
- Zoom Range: The inverse quality indicator since a short zoom range normally means all other quality indicators are better. But if they do make a 18-200mm lens in the future that does get a tick in all the other boxes then it would be almost ideal.
- Aperture: The wider the better (or at least the more versatile). There are some people who would prefer an f/4 lens to an f/2.8 but those are the same people who are afraid of spending a few hours a week in the gym.
Image quality indicators:
- Barrel Distortion: Only a problem on zooms, with large zoom ranges and usually also only on the wide side of things. The 18-70mm for Nikon for instance is pretty nasty below 28mm but fine on every other zoom position.
- Sharpness: Not just centre sharpness, but border sharpness is the key. There's a reason why the 30mm f/1.4 from Sigma scores excellent in centre sharpness tests yet still only gets a 2/5 for image quality in reviews. Regardless of how this lens is handled the border sharpness is poor.
- Vignetting: Only a problem on wide angles, and usually on wide open apertures. This is the amount of darkening that occurs in the corner. It's also more of a problem for lenses designed for APS sensors and not for film lenses mounted on an APS digital camera.
- Chromatic Aberration: Purple fringing and colour smearing in areas where the contrast changes suddenly. The lower the better, especially for purple fringing.
- Bokeh: How are the out of focus areas rendered? Out of focus light sources should be rendered circular not octagonal (for 8 aperture blades), and should not produce a ring around the spot as this causes out of focus areas to draw attention to themselves.
- Flaring: Less of an issue but if you shoot into the sun how good of a photo can you get? Every lens flares, the cheaper the lens the more likely you lose contrast when it happens.
Can't think of anything else, but as said above
www.photozone.de reviews lenses by doing tests on many of these parameters.