The fact that you can jump on and play any web game just like that, without having to run installation, is quite an important consideration IMO. This is, of course, what makes web-based games quite attractive.

Games are different from other software because the burden of content in a game is on the developer. In the case of productivity software (wow, I sound so 1993) the software is a mechanism by which the user may create content. With GMail you create email, with Photoshop you create images, and with Microsoft Office you create macro viruses. This is why you use these applications over and over; your need for content is satisfied by your creation of the content.

Also, with GMail and email clients specifically, you frequently share in other users’ content, so increasing the rapid reusability of the application.

Now, games by contrast are a mechanism by which the user may experience content created by the developer. You could say that the user creates entertainment through his interactions with the developer’s content. Rarely can the user directly create content with a game (Excitebike!) and even in cases where the user creates his own content, the joy of discovery is lost.

The ways in which the world interacts with the player can be considered content. In this way, online games generally have such high replayability because of the constant contribution of the players. The other players interacting with the one player create new content for the player in the form of a new challenge.

This goes back to my original thesis about game design, that given equal development resources, more story tends toward less replayability. If you are playing a game primarily to advance the story, and the story is complete when the game is complete, there’s no reason to play again. By contrast, the games people play thousands of times are generally games without a story, especially those “casual” board and card games. You can study chess and bridge for new tactics and strategies, and thanks to the game mechanics and other players, the game will react in predictable and unpredictable ways. How virtuous a concept! We have set up narrative computer games as an adversary to players which never changes between playthroughs, never better and never worse. Is it any wonder you only feel the need to defeat it once?

Yea I say unto you, replayability is the product of your game’s initial playability (would the player want to even play it once?) and the flexibility of the game’s content (would it be different if the player started again?).