I make 100% of my living using my camera (as well as my clients cameras if they require me to do so). Anywho, AF improvements is crucial to me... the 7d, while not a steller low light camera, I could take in a marginal or dark room and have it nail focus... the 5d2, i have little shot in hell getting my kids in focus when in motion. 1 out of 5 keepers wont cut it... According to chuck w, the 5d3 AND 1dx has more cross sensors than the 1d4 provided you are using modern F4 or faster lenses... even if you are only shooting F5.6 lenses, it has I think close to the same amount as the 1d4. Also the tracking algoithims for the 1d4 is al servo II, 5d3 and 1dx is al servo III... Of course this all is to be determined in real world testing, but from what I heard, that is one heck of an improvement.
ISO, especially High ISO is supposed to be 1-2 stops better... This, of course, will be subject to testing, but for me, I've never been an ISO freak, so I'm happy with as good if not a hair cleaner... What can I say, i'm from the film days so this is head and shoulders better than high ISO back then...
Sensor... not bothered... 21MP is fine for me... It could be the same as the 5d2 for what I could have cared... I've never had a client complaining about resolution, DR, ISO, etc... Those are more personal quirks, but clients typically aren't as discerning.
I could see the arguement of having the raw+jpeg and jpegs shoved on the SD, but i personally would have preferred the 2 CF, but that's not in the cards for now.
What does sell it is customization... MFN button, dual axis level, mode dial lock, better on/off location, better LV/Movie buttons, weather seals... To me those are worth their weight in gold.