I bought a 5D Mark III shortly after they went on sale, and have been fairly satisfied with it, apart from the lack of improvement in sensor technology. It does indeed fix most of the things that were wrong with the 5D Mark II. It seems like Canon just can't resist the urge to put stupid limitations on some features, however. Following are a few of the more egregious examples that I've noticed.
The 5D Mark II had an auto ISO mode, but it was so badly implemented that it was worse than useless. It would adjust the ISO to keep the minimum shutter speed at 1/focal length, e.g. 1/200 second for a 200 mm lens. With modern sensors that's not even fast enough to avoid blurring due to camera shake (although people who don't drink coffee may get better results) and utterly useless for any kind of action photography, which is what you'd typically want auto ISO for.
As I was reading the 5D Mark III manual I came to the heading "Setting the Minimum Shutter Speed for Auto ISO" and thought "Great, they've fixed that stupid design flaw." Alas, it was not to be. The next line explained that you can indeed set the minimum shutter speed - from 1/250 second to 1 second. I honestly can't understand how a camera designer thought 1/250 second is the highest shutter speed that anyone would need. It's not even fast enough to hand-hold a lot of lenses.
A nice feature on the 5D Mark III is the electronic level. Displayed on the rear LCD, it eliminates the need for a spirit level. It can also be displayed in the viewfinder, which makes it a lot easier to keep hand-held shots level. There are two stupid problems with the VF level implementation, however. The first is that there doesn't seem to be a way to turn it on permanently. You have to press a button to turn it on, and it disappears as soon as you half-press the shutter button. The second is that you need to use the custom controls to assign a button to the VF level, and the only ones that you can use are the DOF preview and the M-Fn buttons. Both of these buttons already perform useful functions that I'd really rather not lose. The * button on my camera isn't doing anything, but it won't let me use it.
A useful improvement over the 5D Mark II is that automatic exposure bracketing now supports up to 7 shots, with from 1/3 to 3 stops between each shot. This is of course invaluable for HDR. Having said that, why stop at 7 shots? Nikon apparently allow 9, and I can think of no technical reason not to allow an arbitrary number. This can already be done with some third party remote controls.
Finally, why is the maximum timed exposure limited to 30 seconds? The camera is capable of far longer in bulb mode, and allowing the user to set a longer exposure should be a trivial change. Obviously this isn't a new limitation, but why keep it? Occasionally people do want to expose longer than 30 seconds.
What all these limitations have in common is that they don't appear to save any money, and result entirely from poor software design. They could perhaps be fixed by a firmware update. I have a feeling that Canon will not do so, however.
Perhaps other people would like to share their favourite gripes about their equipment?
