Why would anyone expect to see a big difference in IQ?
It's never been that way, instead we get more features, marginal "IQ" improvement, mainly in the high ISO noise department, and that's about it.
My interest in the MKIV is about getting more resolution, hopefully 1 stop improvement in high ISO noise, nice LCD (LCD on the 1DMKIII is crap by any standard), possibly video...and of course all the other things that typically comes with a 1 series body...build, seal, battery life, form factor, speed, etc.