One of the most powerful features of Julia is that packages (and user code in general) aren't at a major disadvantage to code included in Julia itself. 0.4 promises to make this even better with precompilation and documentation.
Many of the folks who have spearheaded the big packages are also contributors to Julia itself. And many of the core developers help out with package development. So the lines between user, package developer, and Julia developer are all a little blurry. Also note that many packages aren't owned by just one person - organizations have formed to help ensure packages are well-supported. I think the general direction is towards moving more functionality out of base, but to also provide a default set of preinstalled packages. That way the core language remains small, but the default user experience is rich and featureful.
