EC2 is the only provider where you can make API calls to launch 50 servers, have them all booted in a few minutes, then shut them down an hour later. Slicehost's management console will barely load if you have 100 instances, and your servers are all in a single data center. Ditto for Rackspace Cloud. (Both have multiple data centers, but each account is bound to one DC.) I haven't tested Linode beyond booting a single instance on it, but since they bill by the month I assume they're another cloud-ish VPS provider.

Of course, most individuals and small companies would do best to pick a cloud-ish VPS provider. They have much better support than EC2 and small companies usually don't need hundreds of servers.

In my experience Rackspace is good at providing customer support on top of single managed server instances. Their Cloud Sites and Cloud Files offerings (akin to ec2 and s3) feel less than stable, and their management interface encounters unrecoverable errors many times each session. In their Chicago data center they don't allow moving backup snapshots to s3-like storage, all backups are stored with the server. This alone should be a red flag to anyone who is doing any cloud hosting. It just feels like something they're trying to get into, instead of a core competency.

Amazon on the other hand has a much more diverse offering that feels at least somewhat battle tested and polished. They offer you the ability to clone a running server with single click, backups to s3, CDN, block stores, map-reduce, etc. They've also been around for a while, which isn't proof of a good product but it gives you a better feeling that they're serious about the business of cloud hosting (not just hosting).

Not that Amazon doesn't also have its issues. I've heard multiple reports of servers simply "disappearing", which is worrisome.

Having just gone through evaluating various cloud providers for our high-traffic website, this is my personal definition: utility computing as electricity, meaning unlimited (in both directions) metered access. Most "cloud" companies offer neither: a typical VPS provider requires a contract that stipulates exactly how much you may consume, and when your consumption changes, you pay and wait. Imagine if you had to do that at home... a $25 set up fee and 48 hours to turn on an extra light.