According to the vision (and I can only talk about the vision here since the product isn't really out yet), here's a couple of reasons you might consider Azure over EC2.
Azure includes built-in load balancing abilities. If you want to do that in Amazon, you have to roll your own solution or buy a third-party solution like www.RightScale.com.
Azure-friendly-coded apps can be delivered internally or in Microsoft's cloud. If you write apps that have confidential information like financial data or health care data, not all of your clients will be willing to put their data in the public cloud. In that case, they can deploy your apps internally on Windows. That's sold as a skillset win, because you can go from public to private projects. Don't get me wrong - if you master Amazon EC2 development, then you can deploy your apps internally with Linux virtual servers in your datacenter, but it's not as turnkey. (Hard to describe a tech preview as turnkey when it's not licensed yet, hahaha.)
Having said that, it wasn't clear that the load balancing functionality is included in the box with internal deployments. If you have to do a combination of Azure plus ISA Server, that'll be a tougher deployment and management sell.