After a somewhat in-depth conversation about games, what games I've really enjoyed or keep coming back to - all have an element of interestingness. In other words, there is the possibility that something entirely ridiculous could happen. Here are a few examples:
Fable 2 - You have a variety of expressions you could use for any given situation. I've received the title "Chicken Kicker" from the Town Crier after showing enough birds who is boss.
Oblivion - I managed to go through two dungeons without pants on in the game. I don't know how this happened, but it turns out that there were some pants on the floor in the beginning I failed to claim.
I also ticked off a horse so badly that my only choice was a fight to the death. Thankfully I had located some pants, so my death was a tad less embarrassing.
Any game where you can name your character. One of the more recent names was for DragonQuest 9 - I didn't have enough room to write "Mulletude," who was going to be a badass healer with eyes too close together. Instead I had to settle for "Muletude," which actually changed his role a bit. He ended up carrying a lot of the party's equipment, deeming him more "murse" (man purse) than healer. Great!
Multiplayer games also have this element of surprise, and I frequently learn from watching other "real" people (versus AI characters) hack the system.
If you were to extrapolate this concept in real life, you could say that the activity of people watching in big cities and universities is similar. The possibility of novelty is very addictive.