Reporting documentation (unlike SOAP documentation) is not versioned, which is very annoying for developers. Once a new API version is released we lose documentation for the API version we're still using in our code (we have to use the release notes and migration guide to guess what the report fields specification was before).
And since I'm on the topic of documentation problems, I'll complain a bit more: I (and some of my fellow developers too) really don't like the new 2-column table structure. It was much better/easier to use when when had separate columns for "Type", "Display name", "XML attribute", "Behavior", etc.