"I thought INTP's were the serious type."
Rephrase? If you thought INTPs were ALWAYS serious, if that's your definition of a "serious type," you indeed made a wrong assumption.
You may bring your world back in balance quite easily by rephrasing it to say "I know INTPs are capable of intensely focused seriousness." However that still leaves a lot of room for variables, because implied in your further discussion is a context of seriousness as "not centered on self." That is, your paradigm appears to be we should use our gifts on behalf of mankind or something. Am I reading that right?
So let me ask the ultimate INTP question: Why?
World history is speckled with gifted, effective people of possible proto-INTP origins who caused great pain and suffering by deciding to do something good for the world, according to their own definitions of "good." How far back would you like to go? Akenaten and monotheism in ancient Egypt? Charlemagne and nationhood? Hitler? The most recent Bush? How about Nobel and dynamite? Or that good old Mahmoud Ahmadinejad over there in Iran, sure the world will be a better place if he can just get a nuclear bomb in his closet to make sure everyone around him gives him some space?
Great gifts come with great responsibility, because huge consequences are possible if the gifts are applied effectively. It's an extrapolation of the leadersrhip principle: A true leader doesn't make sure people are doing things right; that's mere management. Leadership involves making sure people are doing the right things. That's a whole other ballgame, and thoughtful people are inevitably going to second guess themselves and quite likely end up in a state of permanent ambivalence.
For instance, what about oil? Of course we are going to use it all up, the only question is whether it's in 20 years or 500 years. Some say the responsible thing to do is cut our use for momentary benefits like cheap energy, divert more of it into products that provide longer dividends in terms of extended use (durable plastics and whatnot) and generally postpone the day of reckoning. That's the accepted wisdom on this. Except -- what if the collateral outcome is stagnation of science? What if we focus so hard on prolonging the inevitable that we lose the mental tools and physical plant necessary to deal with an oil-less world? Maybe the thing to do is consume oil faster, so that when it runs out we have six billion people clamoring for science to find a cheap-enough way to extract energy from hydrogen or whatever to replace it, because the absence of sustainable energy would lead to worldwide violence and the collapse of civilization? A great many of our best ideas come out of crisis; WWII carried a huge price in human terms, but it also created scientific leaps in relatively short amounts of time, as nations focused intensely on survival. Ditto space -- the benefit doesnt' seem to be getting into space itself, but the scientific knowledge about stuff that makes life better here.
Maybe we need to make things difficult, like John Kennedy said, because most good things come out of challenges, not complacency or parsimony.
But in any event: After devoting serious thought to many things that defy predictable outcomes due to imponderable variables, sometimes the best alternative might very well be to put on some music and get silly on the INTP forum with others also exhausted by their inability to figure out how to actually save the world or even figure out if the world deserves to be saved.
Hope that was in keeping with the discussion. I've been off indulging myself in Florida, first actual prolonged, away from home vacation in more than 20 years. The vacation included avoiding serious thinking, so I'm kind of overdue.