Here’s a pet peeve of mine, because I see it time and time again: Folks work on software or projects, put in a ton of effort, and then do nothing to promote the project or release. (And, for bonus points, complain that they don’t understand why the project isn’t getting more attention!)
This doesn’t mean developers have to do double-duty as marketeers and public relations folks. Well, not if they can pass the torch onto interested contributors who are happy to do it for them, anyway. It requires a little coordination and effort, but why put all the work into a project and then not get the attention of the users (and potential contributors) you’re trying to reach?
Additionally, it really helps to blog, tweet, and otherwise spread the word about projects while they’re in process. If you want people to collaborate, they really need to know that you’re doing something.
This isn’t necessarily intuitive for folks, I understand. But it is absolutely, vitally, necessary. Maybe, occasionally, a project is just so darn awesome that somebody happens to stumble on it via GitHub or whatever and word of mouth makes it a success – but typically, things get out into the world via consistent updates and communications to the right channels to get the word out.