Seriously, we don't know the full extent of what the 5D3 is going to have, but the speculation of differences is:
1 ) 5D3 will sport a 61pt AF and advanced metering system, but will not be the same as the full pro-level AF and metering of the 1Dx
I seriously hope they don't ruin the camera here and put in some million point AF chip that has the same crappy precision, accuracy, tracking, ability to deal with backlighting, etc. that the rebels, xxD and even the 7D has.
I mean come on they are not upping the MP and the D800 is loaded so this is not the time to play more stupid internal segmentation marketing games!! Jeez they have had a decaded to fiddle with AF and you know maybe if they didn't waste money having to create so many different watered down versions and just focused on making one good system that worked well (ok and maybe a cut down one for Rebels) they wouldn't run into 1D3 AF fiascos and all. Nikon doesn't waste resources finding ways to make 30 different versions of crummy AF and allows one to get top AF without having to buy a brick!
2 ) Will be much lower shoot rate, approx 7 fps vs. 12 fps of the 1Dx (1Dx also has a 14 fps JPEG burst mode)
I really hope all those 6.9fps are correct. If it is say 4.5fps and with water down AF this thing will be dead in the water.
3 ) Will have some level of weather sealing, but not have the weather proof pro body of the 1Dx
most probably so
4 ) Will not have the same level of noise performance at high ISO settings as the 1Dx (1 to 1.5 stops lower performance)
No way. If that is true then it will probably do worse than the 5D2! I doubt it would do more than 1/6th stop worse, if even. It shouldn't unless they majorly messed up or something had to be altered to get top video quality or something.
5 ) 5D3 looks to have dual cards, CF and SD cards, but the 1Dx has dual CF cards
6 ) 5D3 will likely have a smaller RAW buffer (temp storage of images during shooting as the camera xfers to the storage cards)
hopefully they don't go too far with it though and make it useless
7 ) 1Dx has a built-in grip, the 5D3 will likely be an optional purchase
100% for sure an optional purchase
8 ) shutter probably be have a rated duty cycle life lower than the 1Dx
9 ) 5D3 will probably have a few video features the are not in the 1Dx (speculation here, nothing has been reported, it's just a general consensus)
as for additional points:
they will probably leave out the ability to record voice notes along with photos
they will probably fail to outline the histogram again so you won't be able to see where it ends unless you are shooting in a lab
hopefully they will have the 1DX's, mulit-point MFA for zooms (honestly with how great the D800 is Canon just can't play so many stupid little cripple everything but 1 series to pieces games any more, especially not with the sole 1D body now at $7000)
hopefully they will finally have dribbled out enough AutoISO options to finally give us a working version after a decade (by allowing it to use EC now) although I have my doubts
hopefully they don't go with cheap read out electronics for the 5D3 and have it have ugly banded shadows while (perhaps) fixing the 1DX shadows (the 1Ds3 had less shadow banding than the 5D2), if they do then the D800 will also destroy it for low ISO DR despite having more than 50% more MP.