I think you can have a piss poor plot without bad writing though. Most of Bioware's RPGs follow the same basic premise - ancient evil, only you are bad ass enough to stop it, shit gets real, world saved - yet contain some absolutely fantastic writing and general story-telling.
Similarly, you can have bad writing as long as it fits. Gears of War is probably the most common example... the dialogue and the story is ridiculous trite... but it's exactly what you want out of a game of that nature, much the same as Resident Evil - although here is probably a good example of perfect writing poorly executed.
I think a lot of the problem is essentially this;
The Random One said:
Eh, not every one of them. Take The Darkness, for instance. Despite the agonizing opening in which we learn the main character lives in the GTA world and treats shotguns like a ship in a bottle, it's actually a very interesting take on the gritty 90's comics, making every character more humane and making the conflict of being a vessel for utter darkness an actual conflict. I think it's got a pretty good story.
...for a videogame. YEAH I WENT THERE
While not really concerned with The Darkness, the point at the bottom rings true regardless - I played Bioshock and thought the plot was fantastic, the story and the way it played out was brilliant, which is why I can happily agree when it's lauded as such. However, most games follow writing which, yeah, are great - for a videogame.
Somebody in the thread mentioned Zelda games as suffering from atrocious writing, which I quite vehemently disagree with. Sure, this is coming from a self-professed fanboy, but the writing is great, the stories are fine even if the plot is clichéd... but it's only fine for a videogame. You could take Twilight Princess - probably the best story in the series - and put it to paper or the big screen and it'd be ridiculous. Even I wouldn't watch it. (Well, maybe)
A lot has been made on a lot of Escapist columns in that games seem to be trying to tell stories worthy of the most epic of movies, in the style of those movies, forgetting the core thing that sets them apart from the film medium - the interactivity and gameplay aspect.
Do we need both to enjoy a game? No, not really, but it helps in a lot of titles. Imagine playing, I don't know, Grand Theft Auto IV without the storyline. All you have is an open world sandbox with absolutely no reason or drive to move forward, and nowhere to go. Fun? Probably... for a very short time.
I think I've started to lose my point. Basically, I agree that gameplay should come first and foremost. If I want fantastic story I'd read a book or watch a film... if I want interactive entertainment, I'll play a videogame... but a believable story, well executed and fit within the confines of the game itself, shouldn't be too much to ask.