SharePoint15 workflows are now powered by Windows Workflow Foundation 4, which is substantially redesigned from previous versions. One of the new features that I like about SP15 workflow is error logging in the workflow page in UI. When any workflow (OOTB/SPD/VS) errors out or canceled, a nice little tiny icon is shown just next to the status field. When you click it, you get detailed description about the error message along with the correlation id which can be used for further troubleshooting in case required.
In the screen above, it shows the correlation id along with description about the actual error. In this case, its an authentication issue – the user doesn’t have access to start the workflow. Once the issue is fixed, the icon goes off.