I got into transport nursing for the intellectual reason. Even though I have done great things in the ER and ICU's, there is a physician there to say "Yes, that is the correct intervention; go ahead and do it."
I started doing critical care transports, then worked my way up to fixed-wing and rotors. (It took me a little longer to fly, as I was deathly afraid of flying!) I feel I am able to utilize my brain power without having to have someone else order it. I know, standing orders and protocols tell me what to do, but WHICH protocol to use relies on my (and my partner's) brain and assessment abilities. That is also why I enjoyed working in a rural (15-bed) hospital. There was usually no physician on location.
Short answer to a very thought-provoking question.