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The most consistently-replayed computer game in the world has got to be Solitaire, the version of Klondike that is included with Microsoft Windows. So what's its appeal?
Some of these characteristics are helpful to us and some of aren't. Item one, for example, isn't much use. Most of us want to design new games, so computerizing existing games from the real world doesn't have a great deal of appeal to us as designers. (It can have a great deal of appeal to those of us who are AI programmers, however. Many existing games make interesting programming challenges - chess is an extremely simple game, with no randomness and no hidden information, but look how much money has been spent on chess programming!)
Item five, too, doesn't help us much. There's not a lot we can do about the fact that Solitaire is free. Most of us want to get paid, so our games have to sell, and that means that there has to be enough content in them for players to justify opening their wallets. Unfortunately, content is expensive to make, and it often lengthens and complicates games. There's an interesting relationship here, one that I think we can learn from: the most replayable games are also the smallest and cheapest to implement.
Items two, three, and four get to the heart of the matter. Summed up in one word each, they are simplicity, shortness, and ease. Trip Hawkins, the founder of Electronic Arts, used to insist that games be "simple, hot, and deep." Simplicity and depth (i.e. subtlety or variety) both contribute to replayability. By "hot", he meant exciting, which is neither here nor there as far as replayability is concerned; it helps if that's the sort of game you like. Solitaire isn't very exciting, but it's still highly replayable.
Replayability requires a simple, compelling, addictive challenge and the most natural, frictionless user interface possible.
Personally, I don't think Solitaire is a very interesting game. It's too random. You lose far more than you win and no amount of thinking you can do will change that. Free Cell, which also ships with Windows, is a much better game. It takes a little longer to play, but it offers a mental challenge that Solitaire lacks. Its rules are almost as simple and its user interface is identical. And unlike Solitaire, Free Cell rewards patience and persistence; it isn't that hard to solve to begin with, and in fact all but one of the 32,767 deals of Free Cell can be solved with enough effort. The knowledge that it can be done encourages you to continue to try.
Designing for replayability is the purest test of the game designer. Replayability requires a simple, compelling, addictive challenge and the most natural, frictionless user interface possible. All the big, expensive, fun things that we think game development is about - spectacular graphics, hundreds of unit types, fifteen different camera angles, and voiceover narration by Patrick Stewart - are irrelevant. The game is reduced to its barest essentials: the challenge and the means of overcoming it. If I were trying to design a game for high replayability, I might actually start with cards or dominoes, something I can shuffle around on a tabletop. They wouldn't necessarily end up as cards or dominoes in the game; they could end up as genies or giant worms just as well. Their surface appearance doesn't make much difference as long as the gameplay works.
Replayability is not an absolute necessity for computer games. As my friend Jeff Wofford at Deep Red Games points out, many games offer so much gameplay - forty or fifty hours is not uncommon - that a lot of players don't even finish them the first time through, much less play them again and again. If we've given our customers an enjoyable time for a dollar an hour or so, we're doing pretty well; certainly better than the movies do, even if our customers only play the game once. If I were designing a large game, I probably wouldn't worry about it much.
Still, the question of replayability is one that every designer should ask herself in the initial stages of game design. All players, casual or core, want good value for their money. If the game can be played to its conclusion in a few minutes or hours, then you had better either set the price accordingly, or make sure that it's replayable by design.