I almost forgot what the thread was about...
BTW: Doylem, your 18-70 is not what I'd call a "cheap kit" lens. It is a faster lens than most cheap zooms, constructed at a much higher level, has non-rotating front element, metal mount, etc. etc. not to mention the excellent optics. The 18-70 is in my mind one of the affordable "classics" and I actually have been thinking of getting one to supplement my heavier 17-55 -- lighter, more reach. It's a bargain, but it's definitely not "cheap" in my estimation.
To the OP, and his question: I'd say that generally cheap lenses are constructed cheaply out of light plastics, have a fair amount of play in their moving parts, and often even a plastic lens mount. Most of them are variable aperture zooms, and usually don't perform corner to corner very well wide open. Often they are thrown in with a entry level body as a "kit" lens. Optically, some of them are actually quite good (Nikkor 18-55 f/3.5-5.6 comes to mind) but you have to consider using this lens wide open at 55mm only gives you f/5.6. Not good even for cloudy days, really. Most folks will find their image quality to be quite sufficient, especially if viewed on a computer or printed as 4x6 prints.
Expensive lenses usually are the primary brand (Nikon, Canon, etc) or specialty third-party lenses (Leica, Zeiss, etc.) fast primes and zooms (f/2.8 minimun usually,) although some excellent "moderate priced" lenses are only a constant f/4, but build quality is usually similar to the f/2.8 lenses, which makes them good alternatives for photographers who want quality, but either are on a budget or want more portability. You'll find lots of metal construction, often moisture sealing, smooth action and tight tolerances in manufacturing. These are usually targeted at skilled photo enthusiasts and pros, due to cost and the benefits of the big apertures they're capable of. Nice, tight subject isolation, pleasing background blur, fast autofocus, durability, very good performance wide open.. all hallmarks as a rule of expensive lenses. Often better color rendition, contrast, sharpness, bokeh... the things that make you go "Wow!" when you see that certain shot, whether it's a sports action shot or portrait, and wonder why you can't get that kind of image with your "cheap" lens... Those are just some things that come to mind... There are some excellent cheap lenses, but then again, cheap is relative.
Now... back to the flash photography discussion...
