Something I haven't seen a whole lot of discussion of in the videogame developer literature is variance in games. Poker is a high variance game, so there's plenty of discussion of the phenomenon in gambling literature.
I think variance is one of those things that a game designer should give serious thought to when designing, along with feedback and emergence and learning curves.
So what am I talking about?
Variance is the amount of randomness in the outcome of your game. Poker is extremely high-variance: even the best player in the world will lose regularly. Chess and Go are very low-variance: Kasparov will beat me in chess 99,999 times out of 100,000...maybe even more. (Which means if we play one game an hour...let's say ten games a day...I'll need to play for something like 30 years to expect to beat him once...of course, if I play 10,000 games of chess with Kasparov I might get a little better at chess and improve my odds. Okay, digression over.)
How does this apply to videogames?
The SSX series is high variance, whereas the Tony Hawk series is low variance. When I play Tony Hawk, with most of their challenges, if I can beat a challenge once, I can go back and repeat my performance almost every time. This is because of the Tony Hawk rail system is really tight: you can use rails in Tony Hawk to line up shots and create repeat performances. With SSX, on the other hand, slight differences in timing and pressure on the analog stick can cause you to miss the rail completely and end up doing a totally different run than you might have originally planned.
Devil May Cry and Ninja Gaiden have high variance in their boss fights: when I finally beat the first boss of both of these games, I was pretty proud of myself. "I figured out how to beat the boss!" I thought to myself. Later, I went back and tried to beat that same boss again, and failed. Apparently I hadn't "figured it out" - I had simply gotten good enough to beat the boss some small percentage of the time. (Or maybe there was some kind of dynamic difficulty adjustment going on, making the bosses easier and easier until I finally passed them.)
With Zelda, on the other hand, once you've figured out a boss, you can beat it again almost every time.
I've got to go; I will continue this essay later.