Azure and AppEngine are much more closely related than either to EC2.
Both Azure and AppEngine are platform-aware and do the scale-out magic for you; you write your code and they make sure it scales (more or less, I'm certain there's still some application-efficiency/scalability rewriting, but you never worry about individual machines, in theory)
EC2 means machine level administration and custom scale-out/scale-up techniques. Not to mention there are load-balancing concerns when you're in the more-requests-than-a-single-machine-can-handle range, although that's above 500/sec for many applications.
Both Azure and AppEngine are platform-aware and do the scale-out magic for you; you write your code and they make sure it scales (more or less, I'm certain there's still some application-efficiency/scalability rewriting, but you never worry about individual machines, in theory)
EC2 means machine level administration and custom scale-out/scale-up techniques. Not to mention there are load-balancing concerns when you're in the more-requests-than-a-single-machine-can-handle range, although that's above 500/sec for many applications.