I've seen a lot of confusion and misinformation on this forum about what Canon's "L" designation for its EF lenses is supposed to represent. I hear myths repeated and a variety of accusations leveled at others over this single, scarlet letter.
So, here is what Canon says about it in EF Lens Work III, 8th edition, 2006, Chapter 2:
The bright red line engraved on the lens barrel. And an L for "luxury."
The Canon EF lens L series possesses a level of quality sufficiently high to be called professional, designed to include groundbreaking image performance, outstanding operability, and resistance to weather and aging.
"L." This name is reserved only for those few lenses that can meet stringent standards of performance, using fluorite (an artificial crystal), a ground and polished aspherical surface, UD, super UD lenses, or other special optical materials.
Optical design without compromise together with optical theory and precision engineering technologies that are as steeped in tradition as they are cutting edge.
And the result of the relentless pursuit of these ideals is the L series of the Canon EF lenses.
If you read through Chapters 3 and 4, you will find that there is a general rule that the L series lenses follow, in accordance with the above. Every L lens *must* employ one of the above special optical materials. That is to say, if a lens does not have any of these, it is never called L.
(There appears to be a misprint for the 85/1.2L II, as the cross-section diagram shows a caption for aspherical glass but that particular element is not shaded as such.)
However, there are a few (and I really do mean a few) EF lenses that do use one of these special materials but are not given the L designation, presumably because of the other factors mentioned in the quote.
EF-S lenses are never given the L designation, even if they satisfy all of the above criteria.
I realize that this is not going to settle the debate over the L designation; I know there will always be people who want to make L about something they have made up in their own minds, like an all-metal barrel, or full weather sealing, or marketing gibberish. For what it's worth, what I have posted here is what can be definitively determined from Canon's own published materials.