The first one you play for the main story and the D&D rules. It's certainly dry compared to BG2, but i still love it because it has the best ruleset in gaming - D&D! There are a few good side quets though and you should still play it through for BG2, and to better learn the D&D rules.
Damn it, i wish there were more D&D games, the setting and the rules are just so god damn rich comapred to other games. I love that word - D&D, and i will say it again - D&D!
That's both a pro and a con. On one hand, AD&D was an impressive system, on the other hand, it was hardly the most intuitive. I feel like THAC0 can throw some people off since it's different from most systems now. From a pure rules viewpoint, AD&D was pretty wicked cool and creative. The classes alone were extremely imbalanced, hooray for overpowered casters, but it had whacky optional rules like Fighters becoming lords and basically turning into a castle and realm strategy game, the whole Druidic council, and the tables upon tables.
But I digress. I've played a bit of BG, BGII, and Icewind Dale and they are all fun for what they are. If you're going into it looking for Fallout or PS:T, you'll be sorely disappointed. The games have much more complicated combat mechanics and generally more interesting fights but the goal is more of an adventuring tabletop experience rather than PS:T, which was all about the writing and choices. Fallout is somewhere between the two and Icewind Dale is all about combat (it's a hack and slash). And yes, D&D does become more interesting as you become higher level and have access to a broader spell list, which BG follows.
I remember once I was playing Neverwinter Nights, and I thought, "Well, this is okay."
Then right after that I went back and replayed Baldur's Gate, and I thought, "This is so much better than Neverwinter Nights."
Then right after that I went back and replayed Fallout, and I thought, "This is so much better than Baldur's Gate."
Neverwinter Nights was all about the online play where you really could emulate the tabletop experience, though not perfectly. The single-player campaign was duller than a hammer.