Kirstie put it really well.
I have both kinds of bike. The road bike makes me feel graceful as I zoom along at high speed with little effort (relative to mountain biking). If I go with the club it gets exciting when the pace picks up and I'm 6" off someone's back wheel. Other than that it's just the joy of riding such a light, responsive and efficient machine.
My mountain bike is a high-end jobby but it still feels 'sluggish' by comparison. Unlike road cycling, where you can get up to speed and just maintain it with a bit of pedalling, mountain biking is relentless in terms of power input. However, I rarely think about speed (except how to reduce it) when mountain biking on decent trails. With the off-roading, the fun is in overcoming the technical difficulties of certain trails, or hanging on for dear life, or hoping that your front wheel isn't going to land too much earlier than the back as you hit the doubles… Obviously it depends what kind of terrain you have local to you though. If all you've got is fields, it might be a bit boring and the real pleasure might be only the peace and lack of cars. I first fell in love with mountain biking because I enjoyed exploring the local woods. Then I discovered the art of (trying to) ride singletrack fast, and later actually taking the bike into some actual mountains (Lake Garda). To me, it's less about comparing it with road riding, and more about picking out what good things there are off road, be it adrenaline, physical or technical challenges, or just nature and solitude. The answer to your question is that it's head as well as body.
Sorry if I sound like some sort of hippy!