Previously in 0.4 (or so), -D used to show the standard error output of modules. This has been gone for some time, and I've now removed the option.
Replacing it is -v/--verbose, which on /usr/bin/ansible-playbook now shows the data returned from successful commands as well as failures (with the exception of the internal calls to setup, which are very verbose).
You should have better insight into what is happening in your playbook runs.
--Michael