The first capability to understand is the Service Level Agreement (SLA) that the vendor is willing to offer to you. Some of the cloud providers offer virtually no service level commitments whatsoever. While that may be OK for test data, although re-creating a test environment can be a chore too, it certainly does not fit well for a production environment.
Second, look at how you are going to get data to the cloud. File virtualization solutions like those from F5 and AutoVirt that can move file data between multiple vendor's solutions should also be able to move data to either a cloud gateway or eventually directly to the cloud by supporting the cloud vendor's API sets. When it comes to application data however, talk to the software vendors you work with about their cloud strategy. Adding an "Archive to Cloud" button to the applications you use may not only increase the amount of data that can be archived off of primary storage, but it also will increase the quality of the meta-data about that data. Improved meta-data means improved recovery in the future.
Third, try to understand the storage architecture that the vendor is using. That can be difficult. Some providers are not quick to explain what they use. Companies like Amazon, Iron Mountain and Nirvanix have architectures typically invented 100 percent inside the provider. They developed the storage software and deployed it on their own servers and storage. Then there are companies that have leveraged third party cloud software like those from Bycast, ParaScale or Caringo and then deployed it on standard servers and storage. There are also providers that leveraged more traditional forms of NAS or archive storage. Companies like Nexenta and Permabit have partnered with solutions from companies like Mezeo Software to add cloud storage capabilities to more traditional storage systems. Finally there is the complete turnkey approach by EMC's Atmos that combines a clustered storage platform with strong data services, or Cleversafe's object-based dispersed storage system.
Really, an SLA should help you avoid paying attention to all this detail, but the SLA is only good as the company and processes that back it. If you are working for a company you know and trust you may be fine. Otherwise, it is well worth the effort to know what your provider is using and what processes are in place in case something goes wrong.