Cloud computing is essentially the migration from IT products to services, in the same way that consumers of electricity migrated from generators (products) to the grid (services) over a century ago. However, unlike electricity, cloud computing services are generally not fungible.
That is to say, as the cloud provider typically has to store data (virtual machines, documents, database records, etc) in order to provide the service, it is not simply possible to start consuming services from another provider — you need to first export the data from the first provider in an open standard format and import it into the second. Furthermore, in order to automate the process you need both providers to expose an open standard interface. There's no point having no access to transparent data and there's no point having unfettered access to opaque data — users need unfettered access to transparent data.
By using open standards in this fashion it is possible to guarantee important user freedoms while also allowing businesses to make money — this is what we mean when we say "Open Cloud".