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Bookwyrm627: I believe there are a few other minor differences. The only one that I remember noticing was that ToB toned down the OP-ness of the Cloak of Mirroring, making it not reflect spell damage.
I am actually aware of three other differences:
1. Wizard Eyes are invulnerable pre-ToB.
2. Greater Restoration is single-target pre-ToB, but affects the whole party with ToB (apparently, the Enhanced Edition changed this back; go figure)
3. From what I understand, scrolls of Restoration used to cast Greater Restoration, but they now cast Lesser Restoration
Hickory: As I said, you can play the game vanilla. The Watcher's Keep items are only inside Watcher's Keep, and Throne of Bhaal can be skipped entirely. The base game is not changed by ToB, apart from maybe some bug fixes. The bottom line is that with a GOG version you cannot install one without the other.
souldboundxx: Trying to play the original Baldur's gate 2 is more complicated than it should be. Steam doesn't want you playing the originals at all. Gog makes you buy the enhanced edition, and then hides the complete edition with a code. And then you get the complete edition and they make you install throne of baal. Whhhhhyyyyyyyyyyyyyyyyyy. -.- . Next stop, Amazon. Looks like I'm buying this bad boy on disc.
It could not be simpler. It is in no way complicated to play just the original BG2. Bhaal is a different campaign you select. It doesn't effect the original in the slightest.