I love the way that really, really good Techno has this almost unlimited depth to it.
e.g. with a really good track you'll find that each time you listen to it a different element of it shines out, and you end up hearing it in a different way to how you've heard it before.
... and that's before you start mixing different tracks together and seeing how elements of tune A complement elements of tune B.
Infinte posibilities...
Also, coming from more of a Breaks background, I love the way that Techno mixing is a totally different ball game to more traditional mixing, where you mix the beginning of one track into the end of another. Instead, you're kind of dealing with two walls of sound, which you're jamming together creatively to make something new.
It's a completely different way to listen to music.
IMHO.