Physics major here -
Nearly all of med school is memorization. The stuff that is cool is often times not how the body works, but what happens when it fails. Stuff like neglect syndrome where you learn about it and say "really?"
I, personally, thought immuno was mind numbingly boring.
Remember, no one says "yay, I get to go to med school" - you say "yay, I'm gonna be a doctor.". Med school is a means to an ends and it is worth it for most of the people that go. I will probably never be intellectually challenged in the same way I was when I took quantum or thermo, but I'm using my brain to help people and that's rewarding in a better way. I think medicine requires a much, much larger knowledge base than physics/engineering and the problem solving is fundamentally much more straight forward, but you don't always know what details are pertinent. For example, in kinematics, you know that you need to know angle, velocity, accelleration, etc. But if someone comes in with chest pain and a cough, you don't know if they're related. Like I said, I think the level of thought is higher in physics/engineering, but your abilities to determine what is pertinent and to what extent is much more in medicine.
Just my $0.02 - hope that didn't offend anyone. And im def not saying medicine is easy.