As a professional photographer I'm going to have to disagree with many of the people posting on this forum. Many of us need more than 16MP and some of us need even more than 22MP. But with that said, I understand why Canon decided to make the 5D Mark III those specs. Its pretty much everything that a majority of the 5D Mark II users were asking for. An improved AF, better ISO noise reduction, better weather sealing, more FPS. I've also read that HDR option allows 3 photos to be merged within the camera. I think this will be awesome for landscapes and timelapses.
I'm so frustrated by reading post where people say things like "99% of photographers don't need Megapixels". Below I'm going to give you a few examples of why people like me (and there are MANY OF US) need higher Megapixel count.
I currently shoot weddings, product photography and landscapes.
1.) I've had couples ask me for 20x30" print outs, I've even had couples ask me for copies large enough so that they can make wall size canvases out of the final output files. More Megapixels give engagement and wedding couples more options as to what they can do with their photos.
2.) From time to time I'm asked to shoot product photography ranging from electronics to fashion accessories. Currently the 5D Mark II is acceptable for these jobs but I can see clients moving away from what I have to offer eventually if Canon doesn't update pixel count. More Megapixels offers more flexibility in how clients can showcase their products.
3.) I love photographing landscapes. This one is a no-brainer. More megapixels the better. Yes I post my photos on flickr BUT I also sell them and galleries often want these blown up to wall size canvases.
Yes, the obvious answer is to go Medium Format. But in today's economy not all of us can afford a $45,000 Hasselblad H4D and nearly $5000 per lens. I've got a mortgage and car payments so that's out of the question. What I can afford is a $4000 Canon that will offer 45MP and one or two $2000 lenses that will be able to handle the resolution output. I don't need 6 FPS. Better AF, HDR and ISO are definitely welcomed and I'm glad to see it in the new 5D Mark III but that's about it for me. The 5DMIII is still somewhat underwhelming.
If someone wants to call me stupid or a "pixel peeper" because I want more megapixels feel free to email me personally at firstname.lastname@example.org