If it ain't got "Hello World", and it ain't got a cookbook...

It might as well not be downloadable. Open source projects need to be user-friendly to gain acceptance, and if they don't gain acceptance they're doomed to be buried by a rival that is. (Notable exception: if the project is alone in the workspace. Doesn't seem to be many of those around, though.)

Case in point-- the ESB market. Near as I can tell, Mule is running away with that game, largely because of it's ease-of-use model. Anyone reading the help docs can run about a half dozen excellent example configurations not long after downloading Mule. Compare that to some of the other ESBs out there-- they may or may not be able to compete on other merits, but they may never get the chance because they don't get a second look the initial few hours of frustration. Another exception is anything with enough critical mass that it's on everybody's short list sight unseen.)

Examples of doing it right:
Hibernate
Tomcat
Terracotta

Examples of not doing it right:
JBoss WS
Jess
Spring Batch

I'd love to see some of those 'not right' projects do better (especially Spring Batch). Let's see how they fare in the long run-- check back in a year or so.