And while it sounds like the first game has its fair share of memorable characters as well, it seems unlikely it'd be as good in terms of gameplay. In much the same way, I've spent dozens and dozens of hours playing Diablo II, without ever feeling the need to go play Diablo I - if you're used to the second game in which the developers continued the first game's good points while working on its flaws, why would you go back to the first one? Obviously not every sequel in gaming land is better than its predecessor, but in cases where both the original and the sequel are great games (besides the aforementioned ones, Age of Empires comes to mind, Half-Life though I've never played that, and many others), someone used to the sequel has little reason to go back to the original, I'd say.

Though I guess if, after I've played BG2 + ToB a few times (to get all the different romances and some variety in characters and quests and all), I still want more, I could go for BG1.