A stab at clarifying my thinking on this topic:
1. Many smart people with high IQs are not intellectually stimulating.
2. What makes someone stimulating and not just smart? For starters, curiosity and interdisciplinary thinking.
3. There is a difference between someone who intellectually stimulates you and someone who intellectually challenges you. I know someone is stimulating me if I am developing new theories or evolving current ones. I know someone is challenging me if I must stand corrected with some frequency.
4. The most rewarding form of stimulation is when you are pushed and challenged on topics of strength. Sure, I know nothing about cooking, and could probably be stimulated on that topic, along with a million other topics for which I posses zero knowledge. But that wouldn't as exciting as being pushed on a topic where I am already ahead of the curve.
5. In romantic partners, I seek a woman who's stimulatable — someone who is interested. I will be happy if I marry a woman who is very interested in the world, interested in my ideas, and has ideas of her own. Plus all that other (more) important stuff. I do not feel like she has to intellectually challenge me per se on my points of strength; complementary strengths, or non-overlapping strengths, seems like the most workable romantic arrangement. After all, many "power couples" who challenge each other in the same way on the same things do not last very long.
6. Intellectual stimulation is not the be all end all. Other things matter more in relationships and in life. The question, "Do I have fun when I'm with this person?" to me matters more than anything in both friendship and romance, and the fun litmus-test is paramount in life more generally.
(thanks to Steve Dodson for helping me think this through)