I posted this at INTJforums, but after reading a few of your comments on the "INTJ's attractiveness" thread, I thought I'd share it here too.  It was a spur-of-the-moment gift to the love of my life, and it's TRUE!
10)He can find anything.
Me: "honey, where are my glasses?"
INTJ: "beside the bed, behind the water glass, and possibly under the large stuffed turtle that the kids threw there."

9) I can make a joke about transparent aluminum, and he GETS it!

8) He doesn't mind me being a little bit Rachel Ray-plus-Joss Whedon, as long as I don't mind him being a little bit Tony Stark-plus-Mr. Spock.

7) From Please Understand Me: "INTJ's approach sex as a technique to be mastered, and get much gratification from pleasing their partner."

6) INTJ's *love* to master techniques.

5) If I want something different in the relationship, I can just ASK for it. Neither one of us are aware of whatever dumb social rules are *suppossed* to apply, so we don't care. Me: "honey, I wish you would tell me u loved me more often." INTJ: (enters into his phone, "tell ENFP you love her every day by 3pm") "ok."

4) I never have to feel guilty about wanting to learn a new art form: anything I want to learn has some sort of gadget, chemical, or computer program that he can geek out on too. 

3) Reading the ad copy on www.thinkgeek.com can provide us both with hours of free entertainment.

2) Three Words that make our skin tingle: Firefly Date Night.

1) He can say, "I'm not going anywhere," and I know I can believe him

Has any other ENFP noticed that they are so intrigued by INTJs? They are enigmas!  So brooding, intelligent, and private. What is it about them? When asked, can you pin it down?

INTJs and ENFPs intrigue each other because we have similarities, but we're vastly different. We puzzle each other. >.> And both types love a good challenge. INTJs want people to make sense...and ENFPs just want to know everything we possibly can about everything.

I have recently come to realize that INTJs and ENFPs have the same tendency to hide their own intelligence. INTJs do so to avoid being bothered and because they like being underestimated as it puts them at an advantage. ENFPs do so because...we'd rather not be as intelligent as we are. Plus, I think ENFPs are perfectionists at heart, and knowing our own sad shortcomings with procrastination and the like as we do, it's easier to pretend like we're really as naive and simple as we sometimes come across. But INTJs are insightful creatures who notice very quickly that there's wit behind that friendly grin....and ENFPs sense more beneath the surface of the stoic INTJ than is readily visible. ENFPs are rather disarming with our direct, forthright, and genuine manner....some immature ones even coming off as waaaaaaaaaaay too forward and perhaps a bit clingy and desperate for attention. The INTJ is intrigued by this strange mixture of honesty, wit, random humor, and an almost naive outlook on people. >.> I think, over time, it brings out the more protective instincts in the INTJ, lest we poor little generous fools be taken advantage of by the cruel people of the world.

I agree that it has something to do with inherent intelligence, in that INTJs are those who despise stupidity and ENFPs can't get by in so many social circles without being smart enough to keep up conversation. I do know that as soon as I get my INTJs talking about whatever it is that they're an expert at they could go for hours!

The attraction works both ways but it is more likely for the ENFP to make the first step - cause they are the E in the relationship ofc.
The ENFPs and the INTJs are both looking for something interesting and challenging. ENFPs are good at people detection cause they use people`s feelings to understand them, but when they meet the emotionless INTJ they have difficulties understanding their personality as it is only based on logic.
INTJs analyze people when they find the pattern of their behavior based on some logic. As ENFPs are not based on logic they seem random to the INTJ and so they become interested in their personality.
As both types are strong Ns they are always developing and changing and so the fun never stops.

At least that`s my theory (R).