Amazon built AWS. AWS does $5 billion in business. AWS has the variety and depth of services that software engineers expect.

More importantly, it has managed to replicate the entire hosted rack experience without requiring a company employ staff to manage it. Rackspace is nice but they use OpenStack, which still struggles behind the plethora of networking options and protections AWS has - including VPCs, load balancers, firewalls, subnets both private and public, and so on.

There's very little I can't virtualize in AWS that I might find in a typical rack at a co-lo facility, and in that, AWS has no competitors.

(There are other offerings with varying degrees of awesomeness - Softlayer comes to mind - but bang-for-buck, my company still bets on AWS. My only gripe is I/O and the need for local-SSD-fast disk access for clustered KV database stores, which may finally cause us to start looking elsewhere.)

The other thing Amazon has done is build a service that managed to be both extremely complex (meaning I have a lot of control over it), but relatively straightforward for a novice sysadmin to actually use. This is clearly something they are investing a ton of time in these days. The web interface is getting a lot better.