Design report covers the basics of the project, but leaves some questions, and/or is poorly organized and difficult to follow.
Design report has a clear structure which is easy to follow, and covers all the main points about the process and final result of the design. The report has some considerations designed in to support scanning.
Report clearly separates more important information for the reader from additional details that are provided if needed. The document is organized in a way that makes it easy to look for particular information.
The report is indistinguishable from professional work.