OK, let's go over these one by one.
Roguey, feel free to join in.
"Two demons somehow raise a bunch of disparate monster armies and duke it out and also here's a magical artifact from one of RA Salvatore's stories" is more interesting than the Ten Towns' battle against the Legion of the Chimera, whose backstory is much more fleshed out and detailed?
How were they any duller than in IWD1? The villains were certainly more interesting.
long fucking dungeons without variation.
But this is the biggest howler. IWD1's greatest flaw is that it was much more monotonous and poorly paced. Go to dungeon, get to bottom of dungeon, defeat boss, climb back out, return to Kuldahar. Go to dungeon, get to bottom of dungeon, defeat boss, climb back out, return to Kuldahar. Go to dungeon, get to bottom of dungeon, defeat boss, climb back out, return to Kuldahar.
IWD2 kept you moving through the world and through various towns and town-like places instead of sending you back to the same town all the time. Because it didn't rely on one single town for its non-combat "down time", it could afford a variety of interesting non-combat areas like the Black Raven monastery, the time loop puzzle in Dragon's Eye and the ground level of the Severed Hand instead. Some of these weren't always implemented as well as they could have been (personally I think the complaints have been exaggerated) but
they broke up the game's combat slog routine and that was
so important.