I think it's called The Secret because it's meant to be somewhat ironic. If you're talking about using something concrete (the so-called "vision board") as a means of helping you visualize and define your goals, then there's nothing "secret" about it. Speakers like Dale Carnegie and Zig Ziglar have been advocating techniques like these for decades now, because they're very effective at helping people achieve their goals.

But often times, people buy the package and not the product. Therein lies its success: the fact that the ideas were simplified, packaged into a book and documentary, and then taken onto Oprah to put the idea in front of millions of viewers who weren't even looking for it is what made it work so well. Never mind the fact that some parts of it have a pseudo-scientific air to them...the ideas are inspirational and, in the right hands, very empowering.

More importantly, though, The Secret and its so-called Law of Attraction were a runaway hit for a very simple reason: the market for something to believe in is infinite.

(Update: I finally remembered where that final thought came from...thanks, Hugh.)