My two cents (sorry, for my English, it's early morning here and I'm not sure I'm awake already).
The problem isn't about crappy game stories (there're no such thing like a 'quality story' after all), but about modern theory of game design (well, there's no such thing technically since it's too young, but there're some common elements you may found in any textbook) and game development practice that both contradicts storytelling (or narration, it's a better term) as form of expression in art.
The game stories script writers usually underestimate the power of gaming medium, treating it like movies or books (that happened to movies back in the beginning of 20th century, when theatre directors treated new form of media not benefiting from it's advantages, new forms of expression capabilities that couldn't be made at the stage).
Aarseth, game theorist, call games 'ergodic literature' or ' cybertext', referring to the word of 'ergodic' as a completely new form of story perception that's not 'reading' (printed media) or 'watching' (films), but he's just a theorist. There're people in game industry such as David Cage who's asking the right question, i.e. 'what are relations between game and story-telling?', but I'm not sure if he's got the right answer. Anyway a nicely defined task is a half of it's solution.
At this point game stories, in addition to being constructed around wrong ideas as stated above, are pretty shallow by standards of other kinds of medium. They haven't got diversity and complexity of literature and cinema, the cause I believe in target audience. It's not a secret that games are mostly being driven by teenage males (though there new people of different sex and ages are becoming gamers recently but their activity is nothing compared to old guard buying games at faster pace).
Not really a problem, since all entertainment media are driven by teenagers to some extent by numerous reasons I'm not gonna to discuss here, but e.g. cinema potential audience is so huge and diverse, that makes this kind of phenomena universal around the globe, among different kind of people, sexes, ages and culture - so every person able to find something relevant for him\her in the art of cinema. That's not the case with gaming, at least for now.
//On sidenote. Jeez, games even haven't got practically no private (not epic, but private) stories, about, well... love, maybe? The last time I checked that was Mass Effect story I believe. Overall, good game but... nah, pretty standard masculine and immature representation of human relations (well, alien-human, too, but since all aliens in Mass Effect were so two-dimensional and human, ther're completely no difference... hello, to Lem's masterpiece btw). So basically it was a Star Trek\Wars made into a game. Please, get me right, I liked them at the age of 14, they got me interested into more serious sci-fi (not being sci-fi themselves), but I'd like to experience something new.
Ah, I won't continue... it's so saddening. Medium with such a great potential treated like a regular TV series for teenagers in it's storytelling. The sun is rising, I need to get to work.
