Optical quality is more than just sharpness. Distortion is probably the biggest concern for many people after sharpness. Then there are other aspects such as vignetting, colours, rendering style, bokeh, flare resistance, and possibly others that affect our perception of optical quality. Personally, I think I have a pretty low standard of optical quality acceptance, when price of the lens is weighted it. I am easily satisfied. You might think that after using so many lenses, I would become a lens snob, but that's not true at all. I always evaluate a lens from its target audience. One should not expect the same optical quality from a kit lens from those that costs a thousand dollars or more.
Glass Stairs - NEX-5N & E-16mm f2.8. Click for larger.
In the case of the Sony 18-55mm kit lens for NEX, it's on par with most other manufacturer's kit lenses, though I think the original Panasonic 14-45mm (metal mount) is one of the best kit lenses I have used, but they all have their short comings, especially in the distortion department. Manufacturers are increasingly relying on in-camera correction for lens design flaws. I think this is a bad trend, especially when you are paying hundreds of dollars for a nice prime lens; you want a lens that's well designed from the beginning, not a patch up job by software afterwards.
Different kinds of horses - NEX-5N & E-16mm f2.8. Click for larger.