I wholeheartedly agree on the importance of replayability. While it is obviously important to have a good setting, story, interesting characters, etc, as a single player I want to be able to replay the game with as many different character types with varying degrees of alterations in the storyline.
Take, for example, PS:T. A truly remarkable game with a fascinating setting, a story of depth and quality, and characters that were unique and allowed you to believe in them. However, it lacked replayability (which, to it's credit, didn't impact on its overall quality). I played it twice, the second time in a manner that let me get the most out of the story. After that second time through, I knew that I had played it in the most enjoyable possible manner and didn't want to ruin the experience by playing it over and over.
On the otherhand, games like the BGs, IWDs, FOs, MW, NWN, etc, all allowed you to develop very different characters and/or party compositions, if you chose. While none of them could measure up to a PS:T in terms of story, setting, and, arguably, characters, they added a degree of replayability that can keep a player returning for more. As a result, each of those games has the potential to remain a part of a player's leisure time for a much longer period than a game like PS:T would.