I would never have switched to Nikon, but for me, the 5DIII had to be in the same league as the D800.
Specifically, the D800 received the D4's AF and Metering. I expected the 5DIII to received the full AF/Metering of the 1DX. Please do not feed me the "Canon needs to differentiate" line. I am sick of reading that here and there for over 4 years.
I love my 5DII, and there is nothing wrong with it (other than the AF), and I will continue to use it. I have no particular use for more pixels, and I like the 22mp sensor on the 5DIII.
However, OP asked what would have justified an upgrade for me, and in response I say the AF/Metering and small tweaks that would have made 5DIII a match for features of the D800.
Had these features been in the 5DIII, I would have gladly pre-ordered one online and one in my local camera shop (and kept the one that got here first of course, no need for two, can't even afford two) at the $3500 price.
I would have also liked to see a new 24-105 with better optics and the new IS that my 70-200 f/2.8 II has.
On the other hand, IF the 5DIII had the above features, and there was a kit with the new 24-70 II, I would have paid $5000 to $5500 for such a kit.
The new Speedlite is interesting though.