To get anything done, we need to (1) have the desire, (2) turn this desire into action, and (3) continue the action and fight through barriers until we achieve what we want. "Just do it" works well as a mantra to turn the desire into action.
However, after this step, there is a whole lot more. How much natural ability do you have? How hard are you working? How much competition is there in the area? Are there political, legal, or other barriers in the way? There are many factors determining how far you will go. But, getting started is better than not getting started at all. This is an important step that is often not taken.
A more accurate way to put is is that "just do it" is necessary, but not sufficient.
I should say that I just wrote a blog post in the "just do it" camp so it may skew my views:
http://alexshye.wordpress.com/2012/07/10/the-most-important-...