I think you are on to something, Shadowsetzer, but I have a slightly different take. While I played Icewind Dale (w/ Heart of Winter), I have never really had an urge to replay it the same way as for the BG series. Since the engines are the same, it just has to be the story; for me the story in the BGs beats IWD, hands down.
I think that is a big reason that I want to play through IWD2. I have nothing against party games in general, so maybe it was just the IWD story that left me less than sated. The first** RPG games I played were the original TSR Gold Box games ( Pool of Radiance, Curse of the Azure Bonds, Secret of the Silver Blades, and Pools of Darkness), and I liked those party-based games fine (even replayed the first two a year or so ago).
**I played some Wizardry and Ultima games before playing the Gold Box games, but those all run together in my head. Regardless, the Wizardry games were all (or mostly) party based, as well.