The original 100-300L (push-pull) wasn't any shorter when at minimum setting than the new 70-300L (twist) from what I recall (of course true 70mm vs 100m).
I didn't even know there was a 100-300mm f/5.6L! What an odd lens - looks like Canon took the consumer-grade 100-300mm f/5.6 (which is different than the newer 100-300mm f/4.5-5.6 USM lens), and simply added L-level optical elements and a red ring, otherwise keeping the same consumer level build.
yup, that pretty much sums it up
people called it the Meat Grinder
slow and very, very noisy non-USM AF
It was my first L lens, I thinkit sold for like $900 or so new when it came out. I got one for about $300 used back in 2005 or 2006.
One thing it managed was having excellent large scale contrast at 300mm, something that none of the rest ever managed until the Tamron VC and the new 70-300L. Although it had the contrast at 300mm I don't think it had the sharpness of the new L at that long end. The lack of IS was a bit troublesome, the slow and noisy AF not so hot and the f/5.6 even at 100mm a bit rough though. The fluorite element in it, for a long time, had it far an away the best super-compact 300mm quality you could get. I eventually traded it away for 70-300 IS since I decided a bit faster AF and IS just managed to outdo the lesser 300mm large scale contrast. The new 70-300L means no compromises either way, it's definitely better than either of the lens (and tamron) in pretty much every last way.