One of the hallmarks of a good movie is its ability to make the viewer empathise with the characters. This is desirable because a character that the audience has an emotional investment in is one in which they want to know what happens to them. When a movie fails in this, it fails to capture the audience's attention, and so the audience become uninterested in the story. Most forms of story are similar in this way - a book, a television show or even a comic strip lives or dies on its ability to engage the audience.

Computer games are unique in this regard. The main character in a computer game doubles as the player's avatar, their representation in the game world. The abilities and growth of the avatar define how the player can interact with the world, and for the most part the players are directly responsible for that growth. They have made an investment in that character, of time, effort and emotion. (After all, they wouldn't bother if they didn't really care, and the amount of broken controllers in the world attest to how much emotion players can have when things don't go right.)

So, if merely by the act of playing players can have an emotional investment in a character, one of the hallmarks of a good story in other mediums, computer games have a major advantage over anything else as a storytelling medium. This is why players don't mind that stories in computer games are usually hack jobs written by people with no real experience in writing compelling stories, and why even professional authors tend to have problems with the medium - one of the crucial steps in writing a compelling story is not necessary in computer games.

That's not to say you can't screw it up. Star Fox Adventures was marred by incredibly bad writing, which consistently broke the believability of the world and made the main character exceedingly unlikable. I think it was supposed to be witty banter or maybe ironic detachment, but it just didn't work. It was almost like I lhad two different characters - the one I played with, that I liked, and the one in cutscenes, who I hoped would die an excruciating death.

It's not particularly difficult to introduce an element of emotional investment into a game - give players the ability to make an investment into their characters. This can be as simple as in Grand Theft Auto III, where you could make your nameless avatar fitter by having him run, or as complex as Morrowind where almost everything about the character is because of the player. Take advantage of the possibilities afforded by a direct wire into the player's brain, and you'll create a game that stays with the player long after they've finished.