It is a mistake to think that there is an optical difference between the L Series lenses and other Canon lenses of any organised or set, standard kind.

In general, lenses which have "L" badging are truly excellent optically, have especially good build quality, and are designed with aperture and focal length relationships that tend to be uncommon and expensive. But there is no law that says "every L Series lens has to be a good one" - nearly all are, but certainly not every one.

It is even more important to note that the non-L lenses vary a great deal. They range from the cheap and cheerful (18-55, 50/1.8, 28-135, 55-250) through the reasonably priced and half-decent (50/1.4, 18-135, 18-200) to the pretty woeful (75-300, 17-85) to the very, very good (15-85, 70-300/4-5.6) and the simply superb (17-55, 60 macro, 10-22, 100 macro).