The 5D Mark III bears a clear family resemblance to its predecessor, but it's been improved in many ways. Indeed its relationship to the 5D Mark II is much like that between the EOS 7D and EOS 50D - the Mark III just feels like a proper high end camera (finally).
From the front the 5DMark III looks much like its predecessor, with the most obvious change being the large, repositioned DOF preview button. The overall body shape is a bit more rounded and 'organic'.
From the top, the biggest differences (aside from the Mark III's more curvaceous form) are a 7D-like On/Off switch behind the mode dial, and the addition of a customizable M-Fn button beside the shutter release.
The most changes are on the back. The 5D Mark III gains a combined live-view / movie mode switch like the one on the 7D, a 'Q' button providing quick access to the rear-screen control panel, and a separate sliding lock switch for the rear dial. The column of buttons to the left of the 3:2 monitor hints at a slew of functional tweaks in playback mode - more on these later.
Additions/changes vs the 5D Mark II
EOS 7D-like control layout (power switch, rear dial lock lever, live view/movie controller)
Electronic levels display in viewfinder and LCD
Customizable button and control layout
3:2" screen with reduced reflectivity
Locking mode dial
Revised playback mode with side-by-side comparison displays
The 5D Mark III is a similar size to its main competitor the Nikon D800 and as we'd expect from two DSLRs of the same generation its control layout is very similar. Obvious differences are the D800's built-in flash, the exposure mode dial of the 5D Mark III (the D800's exposure modes are accessed via a 'mode' button on the top plate) and the 5D III's rear control dial - a feature of high-end EOS SLRs for more than 20 years.
Both the 5D Mark III and the Nikon D800 are solid, purposeful cameras which carry a reassuring 'heft' which sets them apart from their lower-end stablemates. Note the D800's AF mode switch to the left of the lensmount - like all EOS D/SLRs the 5D Mark III's AF modes are accessed via a button and screen interface.
This view shows the differences in design philosophy between Canon and Nikon very clearly. The 5D Mark III lacks the D800's built-in flash, and features an exposure mode dial on the left side of the top-plate. The D800's exposure modes are changed using the 'mode' button just aft of the shutter release.
All of the same controls, not necessarily in the same place. From the rear the 5D Mark III and D800 are very similar, but arranged slightly differently. The 5D Mark III features a control dial rather than the D800's 4-way controller, and although both cameras have a button to activate live view (and an accompanying switch for still or movie mode operation) Canon places it up by the viewfinder, and Nikon down close to the rear of the body.