As far as I understood it, it's apples and oranges. DotCloud provides specific services, for example, running a Solr cluster, which is a higher level of abstraction really than providing support for a specific programming language.
Also, not all PaaS players will commoditize over the same feature-set. Example in point, Heroku's Java support doesn't allow you to put a .war file up like you can do in Amazon's Elastic Beanstalk.