I posted something along these lines in the Road Cycling forum, but haven't gotten a satisfactory answer, and in my experience folks in the mechanics forum are more knowledgeable so I'm trying here.
As I understand it, the significant improvement of modern road pedals compared to the original Look pedal system is a lower stack height (basically the distance from pedal axle to bottom of shoe). How valuable is this? Is stack height a good reason to upgrade from old Look-style pedals, to a more modern system like Look KEO or Shimano SPD-SL?
(weight is basically a wash, since my Nashbar-branded Look-style pedals are slightly lighter than the Shimano 105 or Ultegra-level SPD-SL pedals)
Note that I'm talking about the original Look "Delta" design that was used by Look until the early 2000's (Look's KEO system debuted in 2004, I think). The Look Delta system was also used by Shimano for a few years in the late 1980's to early 1990's, and is still used in some Nashbar and Performance-branded road pedals.
I understand that there are a lot of viable road pedal systems now and am not here to debate Time vs. Look KEO vs. Shimano SPD-SL vs. Speedplay, etc.