El_Caz: [...] but Baldur's Gate would never be regarded one of the best RPG's if it weren't for it's story.
Maybe that's why it isn't, but it also has more flaws than just the story.
El_Caz: The same thing happens to most other RPG's... if the story fails, the game fails.
If the game fails because of the story, then it did not have much more to offer besides it. It also depends what you deem as a failure because some developpers, like Sir-Tech, purposefully decided to keep it low-key, though they managed to do a fun mix of sci-fi and fantasy in games like Wizardry 8. It's true though that CRPG are one of the best genre to tell a story because their nature should allow for some non-linearity, choices and a different experience for each character, but sadly the medium is rarely used to its full potential.
El_Caz: Gameplay can only contribute to the success and enjoyment of a game if there are other players playing it with you or if the AI is smart enough to keep the game challenging.
Multiplayer is hardly the key factor in enjoyment, unless the game gravitates around it and/or you only like that kind of stuff. AI is also only one element of what makes a challenging gameplay, though it is of course an important one, but it only enters into the question when we talk about combat (combat being large here: it can be physical or on a management scale, etc.) . Some games can offer more than combat, like exploration, ressource management, character building, interactions with the world or NPC, reflex-based challenges, puzzles, etc. Combine all these elements, and more, and you get the gameplay. A good story resting on top of this can definitely improve the whole lot, despite some people who claim that story is superlative in a video game, but they are just as wrong as those who believe that only the story is important and the gameplay only needs to be frictionless enough to let itself be forgotten.
That said, there are a couple of games with mediocre gameplay that I finished only because of the story. One of them was Legacy of Kain: Defiance, and in the end I still felt satisfied, though I probably would have had more fun by watching all the cutscenes together.