It's the first real sport. Just 2 men fighting. It's practical. You don't need any tools or utilities, you just need 2 guys and their bodies.
Also I like the fact that it's about individuals (on the surface, of course we all know how much the coaching and team matters), it's unpredictable, it's violent but at the same time cerebral, I enjoy the fact that there's so many different styles and that each fighter is different from the next - the pesonality of the person influences his fighting style.
I love it that you can look away from the screen for one second and in that second the whole fight can change. This is what initially got me hooked, I believe.
I could go on and on an on, seriously. MMA is awesome.