As I was listening to "The Assalt on Reason" audiobook in my car today I was enjoying envisioning all the types of websites that could be created in response to our current social climate. As I was doing this I realized that much of my life is centered around acquiring a wide variety of knowledge and discusing a variety of ideas with whomever is willing. This is the sort of thing that really stimulates my mind. I also realized that the payoff for me is generating new ideas. These new ideas might include new projects to attempt, new ways of tackling old problems, business ideas, social models, etc..., but the essence of it is new ideas. I think this sort of thing is driven by my primary function (Ne) which is in itself the function for generating ideas. For me I see my reasoning (Ti) as really a tool for figuring out which ideas have merit and are likely to work, but ultimately my reasoning ability (Ti) is there to serve my desire to generate new ideas (Ne).
On the other hand when I speak to INTP's (like those on this site) I gather that they are more interested in pursuing some pure form of knowledge or truth (Ti), and that their ability to generate new ideas (Ne) is generally used in that pursuit of knowledge.
If I had to describe what motivates my INFJ wife it is the pursuit of understanding herself completely, and then using that understanding to grow personally. This understanding and growth comes primarily from the relationships she has with the people around her. I believe this is what you would expect from someone with Ni dominant, Fe auxiliary functions.
So anyway I want to open up the question in general: What stimulates your mind? What is it that motivates you? What types of things really grab your interest and why?