I must be one of the only people that never cared for BG1. I think I played through it once. The Iron Throne storyline was interesting, but I didnt find Sarevok as convincing and menacing a villain as Irenicus.
I also really enjoy the variety of locations that BG2 has, such as the lower planes, the Underdark, as well as the variety of enemies, such as dragons, demons, beholders, mind flayers, etc. A BG1 level party would simply not be strong enough to take on the majority of those.
I enjoyed wandering around the countryside of BG1 for a bit, finding different things, but never found the experience more compelling than that of BG2. BG2 just feels so much more polished, so much more... complete.