Many things are added into ToB : It comes in chapter, that I know of, but you won't be able to return to Amn
( yeah I know...) But, what you gain is a stronghold of some sort ( I won't tell you what or where !
) And the story continue in the land of Thetyr, deeper south from Amn. Small towns/dungeon are added but nothing much compares to Watcher's Keep in size, depth of play and intrigue. That's about it. I don't really know more, ok, a little except that the story seems great and the ending satisfying ! I installed the game a while ago but since it was a long time since I played BG2 and that I had missed many quest and items, I decided to restart from the beginning. I only took a peek to see what was in the game and I can't wait to play it !
As for the changes made when you install ToB on an ongoing BG2 game, nothing much except the Wonderful TAB button that let you see hidden/dropped stash on the map, Watcher's Keep that you can access from anytime in chap 2,3 and 6 and that's about it I think. May be someone else got to see some new change but I didn't notice. Oh, one more thing, It's not a spoiler but a helpful tips : Immediatly go to Watcher's Keep. Why ? Because once you're there, you'll have the option of doing the quest or not ( which I suggest you don't, not when your perfect party isn't formed. reason: LOT'S of exp point awarded, not to be waisted on Yoshimo or some char that won't make the rest of the story ) but because someone will sell you a bag of potion ! This is a real time saver if you are the type of person to stack up on potions. it takes one slot in your inventory space and it carries a LOt of potion ! I hope I didn't tell you too much but just let me tell you that you won't regret it !