dolonjha, I have played all the way through Shadows of Amn several times, so perhaps I can give you some insight about its replayability.
First of all, the things I enjoyed the most were the story and the interaction between the characters. I enjoyed watching how different parties interacted, and I enjoyed playing different characters who ended up in the same situations. Replaying the game also gave me the chance to do things differently than I had before.
For example, in Chapter Two, you can either side with the Shadow Thieves or side with Bodhi in the guild war. That is certainly a big difference. IMO, even a Good character can side with Bodhi, but keep in mind that Keldorn and Mazzy will leave your group forever if you do (the other Good NPC's won't leave).
Really, you get out of it what you put into it. If you let your imagination run free, the game feels completely new if you play a new character. You can also experiment with group dynamics. For example, in one game, I played with a party of Bards. Needless to say, it made the game more challenging since Bards aren't as effective in combat as most of the other classes, but from a role-playing perspective, it was great--it was all about attitude and finesse.
By the time I finished the game, they didn't seem to lack power or ability at all.
For what it's worth, when I first started playing, I always had a full party of six, but once I had seen enough dialogue and character interaction to satisfy me, I started playing with parties made up of only two or three characters. In fact, I just finished playing a game of Baldur's Gate (the original) with just Imoen (a Thief dualled to a Conjurer) and a half-elven Ranger/Cleric. I missed some of the other characters sometimes, but on the other hand, my characters advanced much faster and became extremely powerful since they were so much higher in level. You just have to decide what you like best.
I strongly recommend that you play Throne of Bhaal, for several reasons. First of all, it's a lot of fun. Second of all, it brings a great story to a close. I recommend that you use a mod called the Ascension Patch. It makes some changes to the ending of the game that are a lot more satisfying than the original (without changing the basic premise), mostly by changing the way that certain other children of Bhaal react to your situation.
On a related note, I suggest that you play Baldur's Gate (the original) before playing Throne of Bhaal just because a very special character from BG1 makes a surprising return in Throne of Bhaal. If you want to, you can read the spoiler below; I'll try not to give away too much. But before you read the spoiler, there's just one other thing. Baldur's Gate will feel a lot different to you than Shadows of Amn. There's a lot less dialogue and a lot more aimlessness, and you'll spend a lot of time just trying to cross the maps. In that respect, Shadows of Amn was a lot more compact and took a lot less playing time. But some people like that about Baldur's Gate. Combined with the relative weakness of your character at first level, the game might seem like more of an "adventure". But since you have played Shadows of Amn, you'll already know how to play your character effectively and take advantage of spells, scrolls, and potions, etc. Also, in my own experience, I felt like my character(s) had bonded with Imoen, and I enjoyed having her in the party through all three games. In a way, it's too bad that you already know she's a child of Bhaal, but perhaps you'll be even more fond of her because of that when you play Baldur's Gate.
SPOILER
In the opening scene of Baldur's Gate, you see a very scary individual. Since you have played Shadows of Amn, you might already know who it is: your half-brother Sarevok. He is the mastermind behind a plot to throw the entire Sword Coast into a bloody war. And of course, you're the only one who can stop him. Sarevok overshadows the entire game, and in the final encounter of the game, you have to confront him. Obviously, he is your adversary. But even in the original game of Baldur's Gate, the question arises of whether he is evil just because he has the blood of Bhaal, or whether he would be evil even if he weren't a child of Bhaal. There's even a woman who's in love with him who approaches you and pleads for his life. Throne of Bhaal lets you explore that question, and the answer might surprise you. I thought it was great to see the story come full circle.