The images from the 7D have never looked as good to me as any of the ff (or 1D4), just seemed cruder and coarser - even though they went through the same pp
I've also seen a "certain something" in differences from different bodies and the 7D is not my favorite. To me it has a "gritty" feel even at lower iso and that could be anything. I also have 60D and images from it are similar to the 7D, but ever so slightly different again, even discounting the 7D's more prominent banding in some shots. I like the 60D output nearly as much as my old 40D but the 40D still gives me subtleties in highlight areas I find less "digital" looking.
5D2 also doesn't give me output that looks the way I prefer either, almost always have to tweak tone curves in post. OTOH, I've often been more satisfied with images from my older Digic 3 bodies for that same nebulous reason.
Even the old 5DC, which I didn't have either, was said to produce a very pleasant looking image for whatever reason.
And I guess that's why I'm one of the few (?) who likes to use DPP for post, it tends to maintain that Canon look and feel to the images better than other converters as long as I don't try to make too big of a change. When I need to really push something, it seems DxO and LR do a more credible job but the result no longer has the same feel as a Canon DPP image that was nailed right to start with..
1D and 1Ds stuff has always looked a little better to me too. Unfortunately I don't have any of them to work with or even test. I just don't shoot enough to justify getting one. The 1Dx is something I will consider for the next year or so, IF it can deliver what I want and be worthwhile to me
I think part of the reason is the price you're paying on the 1 series. You are getting the very best they can put into a body. Sensor, support electronics and other hardware and even internal design layout all contribute a little bit to the end result. Sometimes the results are so subtle they're hard to quantify, but the difference in the end result is still tangible. (reminds me of high-end audio gear)
From the horizontal banding and other issues I've seen when looking at the big-picture noise levels I can speculate much of it has to do with minor shortcomings in support electronics, as well as sensor issues. In a 1D body there's a little more physical room to do things better too, from design and engineering perspectives.
I hope the 1Dx performs as well as we hope and builds on the IQ rep of the 1Ds series with some seriously butt-kickin' good AF.