Fantasy video games, like fantasy novels, are fairly substantial sellers in the marketplace. Final Fantasy, StarOcean, and Baldur's Gate are just a few of the bigger titles that immediately come to mind.
An interesting point is that many of the same things fantasy role-playing gamers enjoy are also inherent in fantasy novels -- expansive world-building, stellar characterization, and an engaging storyline are among the characteristics required to make a fantasy video game popular. Unless you're talking about the Final Fantasy series. The spectacularly bad writing/cheesy storylines are part of the appeal for those games. Personally I think it's a case of translation decay -- the games are written in Japanese and then translated to English without (it appears) any sort of rewording of the dialogue.
Games like Dragon Age: Origins, a beautiful dark fantasy from BioWare, are possessed of all the above qualities and become wildly popular as a result. Of course, Dragon Age is also billed as a spiritual successor to Baldur's Gate, one of the biggest selling RPGs in recent history.
What can we writers learn from these video games?
One fact stands out. In non-sports games, story trumps everything. People will tolerate mediocre/cliched writing if the story is spit-shined to a high gleam; heck they'll even spend hours of their time if some of the mechanics are wonky (my wife's complaint about the Dragon Age combat system) so long as the story is fascinating enough.
Don't misunderstand me -- a book written in green crayon on 2-ply toilet paper won't sell even if it's the next Twilight -- and you need to have a basic understanding of good grammar, depth of characterization, and how to evoke emotion, but all those concepts are secondary to having an interesting story.
You can fix practically everything in your writing. Except the lack of an interesting story. Focus on developing that first, and you're well on your way to a big seller.