You most certainly can affect BG's story. It's linear enough to be focussed but there are massive changes that can occur in how it all works out. This is one of the points- you CAN change it to a reasonable extent. FF has given up on that- that's not necessarily a disaster but it is a lower ambition. Lower ambition done well is better than high ambition cocked up, but BG didn't cock it up (and in the case of a game like VIII, FF missed the low ambition as well). For that matter, my biggest criticism of FFX's plot is not its linearity- though that was a shame- but because it completely failed to make sense at the end, and I have never found two people who actually have the same idea about what the plot was, which shows clumsy storytelling.
If you CAN have a genuine, storyline affecting experience, why rule that out as a positive thing? As I say, not to mention the nearly insane detail of the party interaction in BG (especially the sequel) which must give it more replayability than almost any RP game I know. Who lives, who dies, who falls in love, who is redeemed- even what happens to the world- are all up for grabs.
It has a storyline as epic and memorable as any FF game. Its mechanics are good enough, and any faults with those mechanics are only on a par with FF games of its time (and in my experience, not having played the last two, since), it has David freaking Warner as the best video game bad guy I ever played off against... and I could play through the whole thing with my friends!
I'd say maybe you just don't like projecting a version of yourself into the game world, instead of playing a preset character as in FF games... but that Guild Wars thing still gets to me. It cannot just be a random exception- things don't work like that.
I honestly believe that you you gave BG a proper and open minded chance, you'd like it. Though 12 years on, it is late in the day.
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Meanwhile, in regards to XIII- do you guys really feel that its utter linearity and general lack of towns actually improves the experience? You think exploration and discovery have no place in an RP game?
The game sounds like one giant dungeon crawl to me.