For me, two things. One are the kerbals themselves, taking them on adventures, and watching their little smiley (or screaming) faces as they go. It provokes the imagination. Second, is the shear versatility. One day I'm flying an elaborate probe mission to Jool, next, tweaking a space shuttle, and learning to glide to a landing. Next, playing around with boats and cranes. All in the one game. Plus, it's your own goals, and own achievement. Nothing arbitrary here.