Programmable Web just posted a really nice list of 10 reasons why your API sucks. Or doesn’t, depending on how good your documentation and developer experience (DX? Do we really need another bullshit acronym, people?) are. The post also links to their own list of 5 attributes of a great API. Definitely check out the presentation slides in that post.
Note that a number of the bad things (unexpected and undocumented releases, poor developer experience, inadequate support, and poor documentation) and good things (provide great developer support) are documentation-related. How surprising. Yet another bunch of reasons you should add a good writer to your development team.