Heh; I'll copy-paste my comments on the topic from last year or so.
First, I didn't feel like it had any legitimate cause in earlier editions. The notion that demons and devils would wage multi-planar war due solely to alignment differences never sat right with me; it didn't feel like a legitimate motivation.
Second, it felt like it was too often used as nothing
but an excuse for why the fiends hadn't taken over.
But those are both minor reasons. My
major dislikes are:
It came
this close to utterly ruining the very fundamental concepts behind the two races of fiends. It made the demons far too much of a coherent, unified faction. The notion of demons cooperating for
anything in such vast numbers runs counter to their existence as embodiments of chaotic evil.
Similarly, devils are schemers and corruptors. Having them focus so much attention on other fiends drastically weakened their innate concept as a driving force of evil among mortals.
And then there's the fact that both of the above play into one of my major problems with the Planescape setting as a whole. (Let me be clear: I
liked Planescape. I ran campaigns in Planescape. But that doesn't mean there weren't aspects I hated, and this is one.) The entire setting humanized the fiends
far too much.
Demons and devils are the embodiments of evil (albeit different kinds of evil from each other). They're not just evil humans with funky skin and powers; they are truly alien entities for whom evil is wrapped up in their very nature.
Planescape turned them into the D&D equivalent of Star Trek aliens, IMO. It made them far too human, made their cultures and their mindsets far too comprehensible. The notion of a demon or a devil sitting down for a drink in a tavern in Sigil (just for example) is absolutely anathema to what these creatures
are. The Blood War was just one element of what was, to me, Planescape's mismanaging of the very concept of the fiends.