I've been impressed by this as well (not just in the blender project). I think having a good leader who can maintain a clear architecture for the project has a huge impact on the consistency of the result. It can be expressed as something like coding style or source tree layout, but also affect which features make it in, and how they affect a user's workflow.

I don't know how they manage it, exactly, but it contrasts quite a bit with other projects I've seen that feel more haphazard, get-any-feature-in-and-we'll-polish-it-up-later. With blender, things feel polished long before we see them in the release.