I'll also add that the 4 stop IS is a big improvement. Many people still don't like to use IS but I can tell you when using the 600mm on a windy day, the IS stops the wind shakes 100%. Just takes the wind effect out completely. The IS on the MK1 lenses was pretty good at it as well but the new generation IS is WAY better.
Like you said it's an incremental increase on a number of fronts EXCEPT price! I think most of the price increase has to do with more fluorite lens elements being used. That is a considerable part of the cost associated with the new MKII lenses.
Some of the incremental improvements:
Less CA through use of more Flourite elements
Better IS (better stability and additional IS mode)
And probably something most overlooked is the addition of AF feedback with newer cameras like the 5D3 and 1DX to improve focus accuracy.
It's all incremental but together it makes for a desirable combination. I would definitely upgrade some of my MK1 lenses though if I had the cash to do so.
Maybe people are reading too much into fanboy reviews.
The new Mk2 Superteles are undoubtedly better than their predecessors but these improvements are incremental and not dramatic - with the exception of the very welcome weight reduction!
When I purchased my 800 the 600 Mk2 was not an option as it was twice the cost! Yes I would prefer a 600 mk2 but is it worth twice what I paid for a mint 800? I have not had the opportunity to try the 600 Mk2 in the field (though I have had a play in a shop) but I have (as stated above) tried the 500 Mk2 in the field and it is excellent, though it certainly did not put my Mk1 lenses to shame or make me want to upgrade due to the silly prices.