Having owned the 5D Mark II and the classic 5D before it, I can say that Canon has remained true to the 5D ideals of full frame and high quality images at an affordable price (for the professional). The 5D Mark III will instantly feel familiar in the hands of one who has used the 5D or 5D Mark II.
Fast and flexible focus system; excellent low light / high ISO performance; high resolution and excellent quality images
The auto focus points in the viewfinder are difficult to see in the dark; Sometimes the new flexibility of being able to positing your focus point just about anywhere is a weakness instead of a strength as you can't remember where you placed it and can't see well enough in the dark to know where it...