In addition to docs, I’d also add evangelism as often missing. For a project to be successful, someone fronting the project and engaging with a potential community on how it could be used. Call it “sales” or “marketing” even, but open source projects need it as much as proprietary products...
A lot of proprietary projects don't even do that. The project homepage is targeted at managers so to a programmer it often comes across as meaningless marketing blurb (IMHO).
I really appreciated the "drawbacks" link on the sidebar at migadu.com. If you know there are use cases that don't fit your goals, admit them up front.