Dude, I have no idea HOW you forgive that game. Not only was it boring and pedantic, but that plot twist, as Penny Arcade so eloquently puts it, fucking MUGGED the player.
Also, some of the most annoying characters ever, prior to SO4.
Because it's a video game, not a book or movie. The right plot twist can totally fuck up a book, the purpose of which is mainly to tell a story, and to a similar degree a movie that would then need to fall back on pure spectacle, and that can only go so far. For a video game, unless it's something like Phoenix Wright good enough gameplay is always more important than a good enough story, especially when you can zoom past a bad story or it's largely inoffensive and more something that on reflection is really damn dumb (Fallout 3's similar in that regard). Speaking of which, it does have some really obnoxious characters BUT none (or almost none) are part of the core cast, so you'd only see them a few times, and even recruitable ones simply won't pop up in story sequences since it's easier for VA/writing/scene design to only account for those that are always there. Hell, part of it's just bad VA performances, Farleen would've been an inoffensive, forgettable character otherwise, much like her partner.
Though you could also mean the GAMEPLAY is boring, which... well, different strokes.
EDIT: Another angle to bring up actually: I remember enjoying the game long before that plot twist, and still had fun afterwards. As an extension of that "it's a game" point I mentioned, I'm not going forget and ignore that simply because of one dumbass twist.