Im a bit late, but Id like to add my 0.2c to the topic.
In regards to IWD & BG: Both games' (IWD & BG) storylines can probably summed up in a sentence or two...its just that Baldur has much more dialogue and -related quests than Icewind Dale, so it seems to be less about combat.
I dont think Icewind Dale's Story is less complex or interesting than Baldur's Gate's; its just not presented as dramatically and is kept more in the background of things (and it doesnt expand upon a whole series of games).
To me, this is indeed one of the reasons why I prefer Icewind Dale (and Heart Of Winter + Trials Of The Luremaster) to its sequel and the Baldur's Gate series.
I simply find it to be much more interesting to play through the "backyard" of Fâerun instead of having to deal with a "major" storyline involving the settings pantheon and meeting lots of cameos (Drizzt, Elminster etc). This got probably to do with me not liking the Forgotten Realms setting.
Of course, to others, this is one of the selling points for Baldur's Gate.
Anyway, I derail; this isnt about Baldur's Gate vs. Icewind Dale, but about the Icewind Dale series itself.
So how do I see them?
Well, as Ive already written, I prefer the predecessor over its sequel.
This is partly due to the things already mentioned by others, i.e. weird villain dialogues, which are absent in Icewind Dale.
I also somehow like the 2nd Edition AD&D rules more than the new ones, perhaps because I tend to focus more on the characters and the game itself instead of the mechanics behind it ("which feats are the most useful?") and the balancing.
Anyway, overall I think Icewind Dale has a much more intense atmosphere than its sequel: Everything (locations, enemies, music) fits together and feels more "nordic" than in Icewind Dale 2. This has probably to do with Icewind Dale 2's plot that leads you to many different locations (underdark, Targos, chult, fellwood etc) instead of focusing more on the spine of the world and its dungeons. In this case, I think less variation is actualy a good thing and helps to create a specific atmosphere.
Thats not to say that Icewind Dale doesnt have variety! The Dragon's Eye, forlorn mountain temples, ancient elven and dwarven ruins, glaciers, barbarian camps, burial isles etc are all there - but those are locations that are more tuned to the spirit of the place instead of having variety for variety's sake.
On the same note, the encounters in Icewind Dale are more interesting to me, because they, too, fit the setting (see a pattern here?) and somehow "make sense", i.e. they are for the most time not just a bunch of seemingly random selected monsters that are thrown at the party, like Icewind Dale 2 sometimes does (but there it is perhaps partly due to the storyline).
Besides that, I find Icewind Dale 2 to be much more "over the top" in regards to magic and fantasy, which, if you are someone who dislikes "high magic" settings like me, is not something you can look forward to.
I dont know if this is at least partly because of the 3rd Edition D&D rules, the inclusion of subraces (both are absent from Icewind Dale) or simply the designer's decisions to "top" its prequel...but you wont find as much teleporting goblin hordes, demons, dragons and such in the first game. Instead its more about battling orcs, trolls, undead, yuan-ti and the like. One could say its more "classic dungeon crawl" in this aspect. At least in the beginning.
Oh yeah, there are also fewer outdoor locations than in Icewind Dale 2, but not that many.
The storyline itself is much more local instead of Icewind Dale 2's epic "save the world from the gathered armies of the enemies". Of course, the fate of the north is still in your hands - but this time, the enemy is much more subtle in his scheming.
If you include the two expansion packs for Icewind Dale Id also say its bigger than its sequel. At least if I remember correctly.
All that said, I actually prefer Icewind Dale 2's interface and some minor things like the "useable landscape" (clicking on timbers frex), the spell icons etc.
Ok, so this was longer than I thought it would be...
As you may have noticed, my rambling concentrated much more on the "feel" of the games instead of the actual gameplay (difficulty, number of spells, balancing etc). This is just because I set my priorities this way and I also think other people can, and have already, give you more advice on those other parts.
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