While one cloud provider might offer cheap computing or storage resources they may make up for it by charging an arm-and-a-leg for bandwidth. Also, businesses should be aware of higher costs from some cloud providers that offer storage intensive applications.
Performance Cloud providers deliver different application performance results based on geographic location and cloud platform architecture. What’s not so clear, however, is that storage IO can really vary from one cloud provider to another.
Security & Assurance. Your future cloud provider might host your data in a SAS70 Type II data center facility, but that doesn’t necessarily mean that provider has any safeguards in place to protect your data. You may need to roll your own security infrastructure to firewall off your applications or encrypt your data.
Service Level Agreements. A service level agreement (SLA) is one way to gauge a cloud provider’s comfort level with its service delivery platform.
Support. At this point in the game, cloud providers aren’t known to offer great support, but that is starting to change. Today, enterprise officers need to know they can contact someone at their cloud provider when they experience problems.
http://www.datacenterknowledge.com/archives/2010/06/01/how-to-evaluate-cloud-computing-providers/