There's something about it that brings me back to it again and again, the same way I end up playing through Ocarina of Time again year after year. Perhaps it's the fact that I've yet to truely finish either game, although I get closer and closer with each play through and find more and more heart pieces.
Will's playing of Twilight Princess has made me rather nostalgic. Since I am currently Wii-less (preordered it late), I'll probably spend the next few days crusing through Ocarina of Time on the ol' N64.
I've always thought that the mark of an excellent game is when you keep on playing it again and again, many years after its release. As an example, my current set of games are Wipeout64, Unreal Tournament (the original), Ricochet, Worms 2 (and variations thereof), Beyond Good and Evil, and Worms Pinball. Only one of those was released in the past 4 years.