Blue
Ravenous Bugblatter Beast of Traal
This was exactly why it was a hot mess - because to challenge that required DMs to put player-level attention to detail when building NPCs and other foes. If a player puts a dozen hours into their build, and a DM needs to do that times 4-5 per encounter, that's a ridiculous amount of prep. Sure, they could throw straight casters with 9th level spells at PCs with 5th level spells to have an easier time to buidl, but then we're breakign all that versimlitude that the simulation-heavy rules were giving us.The prestige class and feat bloat is what I loved most about the game. Not the power of it, but the versatility that enabled me to create virtually any concept I could imagine. Having only the base classes with very few subclasses means that I sometimes have to sacrifice my vision of my character and cludge into subclass. Since I hate to do that, I have to toss out characters that I want to play and limit myself to what's available.
TL;DR: The same thing the players loved made it hell to DM.