It is boring. That's why I speculated about there actually being a culprit and that someone being our next major enemy.
But that planet isn't Azeroth. The three worlds we were shown are both present in our universe in the way they were shown to us (it wasn't a glimpse of the future, but of the present). Azeroth isn't frozen, which means that frozen planet is a different world. So it's not the Lich King. It's either someone we don't know, or it's no one - just a natural process (like the guy above said).