The ability to accept being wrong
Four pages worth of comments... very entertaining to read. Most of my points have been made already.
I think part of what makes a truly intelligent person is the ability to perceive when a mistake is made. It's all well and good to have great ideas, but if you're not able to discern when those great ideas aren't working out then intelligent you are not. I'll add to my description of intelligence the ability to accept ideas from other people and to recognize when those ideas are superior to your own.
There are extremists on both sides who are deeply entrenched and unwilling to consider different points of view, let alone compromise. You'll recognize them because right from go any argument turns personal. Anybody who disagrees with their points of view are immediately branded an enemy and not worthy of basic human decency and respect. These are the truly stupid people and the ones who represent a real danger to humanity. You can find them on the extreme right and left. No side has a monopoly.