Ben Hutcherson , Oct 28, 2009; 10:22 p.m.
All "L" lenses use some sort of special lens element. Conventional lenses(i.e. non-L lenses) tend to use conventional spherical, crown glass elements.
L lenses tend to use aspherical elements(as is the case for the 50mm 1.2L) to reduce spherical abberation, or fluorite or low dispersion glass to reduce chromatic abberation in really long focal length lenses.
L lenses are not necessarily better perfomers than their non-L counterparts-the L lenses are just generally faster. The 24mm f2, for example, gives better performance at all apetures than does the 24mm 1.2L.
The price, I think, on the used market is due to the fact that there's still quite a bit of stigma attached to L lenses. Most people will still pay a premium for them.