If you can't tell the difference, then what's the difference? Why
not get the non-L? (Serious question, not trying to be sarcastic.)
Well... that's an insightful question... here goes. If you take pictures of objects with the sun directlly behind you all day I don't suppose one could tell the difference because there is no challenge for the lens. Any situation involving some degree of imperfect lighting or high or challenging contrast the L-always wins.
The difficult thing about desribing a good photo is trying to quantify what you like about it. In my case with the L-lenses it is how they render low contrast or poorly lit areas of the photo. (No, I don't own one.) The accuracy with which they do this makes or breaks the photo, at least for me, which is why I think there is something that the focus charts do not show, but can be seen. Something like a 'greyscale sensitivity' rating for HDTV monitors, it's known that the best monitors are still conventional sets for example Plasma monitors have trouble rendering greyscales, but do punch your eyes out with bright colors.
I have seen a picture of a bird in flight on this website, and the picture was so completely realistic, not because of the bright colors, the stunning focus, etc. it was the subtle things i.e. the last 5% difference in the picture that make a 50% difference in your impression of it. In this case it was the fact that although the bird was rendered well, the underside of the bird, and the gray-ish yellow feet it had, and other areas not directly lit by the sun had every little bit of (gray)-scale of contrast and detail so perfectly rendered it just made you study it. None of the detail was washed out or lost regardless of how well or how poorly lit it was around the subject.
There was another comparison set of photos I remember seeing that were of a lighthouse on some rocks on an overcast day, and the standard lens produced a really dreary picture, i.e. just a photo. However the L-lens rendered the low contrast areas of the photo in such nice detail and rich color again, it made you want to study it, it brought real character and detail to the subtle colors in the rocks and made an otherwise dreary photo practically a portrait of subtle colors.
That's why... It reminds me of many things that people instinctively like but cannot quickly explain why. There are real reasons for it (like nice cars, homes with certian proportions, etc) where there are definite reasons but without metrics to quantify them. In the case of the L lenses I believe it's there ability to render fine details in low contrast or poorly lit areas.
