Games are about experience now. That's why they keep getting easier and prettier. But they also mustn't be too easy, or it's more like watching a movie that you have to press buttons for so it moves on. A certain challenge adds to the experience, it creates suspense.
The other half is everything around the challenge. It actually goes hand-in-hand. Suspense isn't really suspense if you don't care about beating the level, so there needs to be a goal, ergo: a plot. Everything else, graphics, sounds, and so on, need to build on that.