Some more high level information about what to expect when building SharePoint 2013 workflows. Below are all the available actions:
The most interesting one for me personally is the 'Call HTTP web service' action. So when you want to write custom logic, instead of writing custom workflow actions just reuse or build a new workflow and call it with parameters if needed. The result given by the web service can then be captured by your workflow.
To conclude: I think this might be a very good alternative for Nintex Workflow.