If I had to ask...

Intelligence: one part ability to think things through, weighing the evidence, and one part ability to read the audience. Put them together and you can effectively voice ideas. Not enough of the former and you're a bluffer; but what if you don't have enough of the latter?

Updated 9 Feb with:

Success is roughly 10% about what you know and 90% about how you present your case....
- Anonymous (cited on a toastmaster's ad)

Great. Thanks Toastmasters, that helps a lot.