dotCloud provided infrastructure (compute nodes with memory/cpu limits), which they wanted to quickly distance from. Docker Cloud is BYOI (bring your own infrastructure) such as Digital Ocean, AWS, Azure, Softlayer. It's basically Tutum out of beta, similar to CoreOS tectonic but less enterprise targetted (pricing upfront, freemium, can use $5/mo DO droplets). It's a managed control panel essentially. CPAS - Control panel as a service?
I liked Tutum (and thus Docker Cloud) as a rapid path to get up and running, but the higher level concepts of Kubernetes appeal more to me for real worl projects.. (note: Kubernetes uses docker and docker images)