I'm just wondering. 



One significant difference is that L series zooms tend to open to an identical aperture at all focal lengths, whereas non L zooms have a variable "fastest stop" depending on where you are in the focal length.
For example, the 70-200mm L is f2.8 across it's entire range.
A non L zoom could be more like a 3.5-5.6 (5.6 at the long/short ends, and 3.5 in the middle typically.) Sorry I don't have a specific lens example for this as I do with the L.