I usually quit most RPGs after a few hours, but I still play Skyrim because crazy stuff happens all the time. The best stories in Skyrim happen in a completely random way, only possible because the game engine is so damn great. It keeps surprising me with amazing and unique situations.
Once I lost my horse in a snowstorm while attacking a witch hut. I went back to a village, slept and my horse didn't come back. I figured a witch killed it, so I headed to the witch hut again to see if I could find any traces of it. When I got there, I could see some lights flashing in the distance, among the snow. Well, my horse was just finishing off the last witch of the bunch, which was desperately attacking it in an attempt to survive.
I think more games could go the Skyrim route and keep surprising players even after long hours. I don't necessarily want the next Zelda to be an open world game, but it would be great if the game engine was able to create unique, random and surprising situations to keep me interested until the end.