Game reviews almost always mention the concept of "replay value". Since video games are fairly expensive consumer products it makes sense that gamers want to feel like they're getting good value for the money they put in, but what exactly is replay value anyway? I'm going to try to outline the kinds of things that are typically viewed as providing "replay value", but then I'm going to finish up by explaining where I think replay value really comes from, which is a good deal different.
Standard Notions of Replay Value
Branching story paths - this is believed to give a game replay value under the assumption that players will want to go back and see how things might have alternately turned out if they had made different choices. This applies almost exclusively to western RPGs, though some newer adventure games like Heavy Rain fit this mold as well.
Multiplayer modes - this is believed to give a game replay value under the assumption that human opponents are less predictable than AI opponents, so players will have novel interactions frequently and not be able to get by on rote learning. Because the experience is constantly fresh, players won't grow tired of the experience.
Different classes/skill trees - this is believed to give a game replay value under the assumption that players will go back and play a game again using a different character class or set of abilities. In this way a game has replay value because players can go back and experience the same set of linear content using a different set of abilities, thus providing fresh challenges.
Random levels - this is believed to give a game replay value under the assumption that a player will have to develop new strategies on the fly each time they play due to things like level layout and available resources. Think of this as a kind of automated version of the appeal of multiplayer content.
What's Wrong With That?
There are two problems with viewing replay value in this way. The first is that, regardless of how much variety these experiences are purported to offer, the actual gameplay will be more similar than it is different. Individual players may sometimes act in unexpected ways in an online FPS, but there are always dominant strategies, and these tend to be repeatable. This is true of a game with random level/resource generation as well.
The second is that, in truth, replay value is only one thing - it is a simple question that players ask themselves after they've finished a game. That question is "how much did I enjoy that experience?" That's it. If a player enjoyed the experience enough then they'll want to have it again. If they didn't enjoy it very much, no amount of variety or novel interactions is going to get them to go back and do it all over again.
Allow me a personal example to try to illustrate why this is. Metal Gear Solid is a clearly defined, linear game. The player always traverses through the same rooms in the same order. The enemies are always in the same places, and they always have the same patrol routes. The story is always the same, aside from one minor change made to the ending depending on whether or not the player successfully completes one particular sequence. And yet I have played Metal Gear Solid from beginning to end probably about ten times. Why have I done this? Because the experience of playing the game is exceptionally compelling. Branching paths, random levels, etc. would not make me play the game any more. I've played it many times because it is a great game. Greatness, not variety, is what gives a game replay value.
Let's Talk About Sports For A Minute
Perhaps this is most easily demonstrated by looking at sports games. Football and hockey are the primary sports I'm interested in, so I'll talk about those, but in my experience this is true of all sports games and, I think, to most games in any genre.
When I play sports games I almost always play in franchise/general manager mode, where the goal is to play as the same team over the course of multiple seasons, managing the roster as well as playing the games. I recently saw a statistic that for one major sports game (EA's NCAA game, I think?) something like 3/4 of players primarily play the single-player franchise mode, as I do. For a football game, this means playing between 16-20 games per season; for hockey it can be upwards of 100.
The interesting thing about a sports game is that the gameplay doesn't actually change much between games. It's entirely possible that I'll play a few games of hockey in a row where I'll win 4-2, usually with the same players doing most of the scoring. In a football game I'll almost always choose a running play up the middle on first down. As explained above, I'll do some of these things literally hundreds of times over the course of a season. So why do I (and others) keep coming back to it?
The reason we keep doing it is that the mechanics of playing hockey or football (or an online shooter, etc.) are compelling. That's what gives sports games replay value - the actual process of playing them is enjoyable, regardless of whether they're novel. And while the underlying mechanics at play are very different in other genres, ultimately it's the same thing that will get players to play again - how much did I enjoy this experience?