With all respect, I don't think cameras or focusing systems, or even lenses make "soft" pictures- the photographer does. Every camera has limitations that need to be learned & mastered. It's just a tool at the end of the day.
Yes indeed, that's why I've determined that the 5D Mark III belongs in a controlled environment like a studio whereas I'm waiting for the two 1DXes I ordered on announcement day for my event and concert shooting. That seems to be the 5D Mark III's limitations.
I only tried out the 5D Mark III last night because...
1. I thought it might be able to handle it given it has basically the same AF as the 1DX and...
2. My 1DXes have been delayed so I have to shoot with what I have.
I'm like you have a 1DIV and have put it down since my 5DM3 has arrived. I have tried to put it to the test (low light, sports, engagement shoots...ect) and intially was dissappointed. I wasn't quite nailing focus, the right exposures and trying to find the correct "case" that would suit my shooting style/subject.
I went back to the drawing board, read the 5DM3 manual over and sat down to really understand the new AF and AI servo features and watch a bunch of the new videos on the Canon learning site about the new 61pt AF.
I quickly found out, the the 5DM3 AF point-linked spot metering was ONLY limited to the center point, unlike the 1DIV that had been accustomed to, so my metering was always off. Switching AF points on the 5DM3 required a extra step, unlike the 1DIV where you can use the toggle stick at anytime. Also, for some strange reason, my lenses were a tad off and I needed to micro adjust. My best lens, the 35L 1.4 was spot on perfect with my IV, but on the M3, I need to micro adjust +5. My 24, 85, 70-200 were spot on. I played around trying different cases and it turned out using the default case and slowing down the tracking priority seemed to do the trick.
Just these small tweaks and a lot of practice as rendered some great images in different enviroments. There is definate learning curve and adjustment period. With all these people that say, my 5DM3 shoots nothing but great images, is just an exaggeration, because nobody has 100% keepers.