In research for my book Evolve!, I identified three primary sources of motivation in high-innovation companies: mastery, membership, and meaning. Another M, money, turned out to be a distant fourth. This sounds very similar to Dan Pink "Autonomy, Mastery and Purpose" as outlined in his book and in this great RSA Animate that forwardslash posted a while back.
There's no doubt that these things are what make people "happy" in their careers. Money doesn't do it, it really doesn't. You have to feel like you are creating something, working for a greater good or goal. When it's just for money, it's a pretty shallow and short lived sense of meaning.