Oh, there was also a heap of hard to measure things, too:
Vassal of Bahamut required you to kill an Adult Dragon. Alone. Not a real problem, but how many gaming groups are gonna let the Paladin go and solo the adventure's capstone boss so that he can enter a prestige class? Hell, how many Paladins can manage that feat without help at a level that makes the class worth entering (it's 10 levels long and most of the juice is towards the end). Pretty much had to resort to it being "in your backstory" which means that it may as well not exist.
Stormlord requires you to be struck by natural lightning and survive. So, considering only about 400 people are struck by lightning each year, you have a pretty slim chance of ever becoming a Stormlord. Which means players end up standing on a hill in full plate waving a steel rod around in every storm.
Scorpion Lord (or Heritator, maybe?) requires you to be stung by an adult giant scorpion and live. You end up provoking scorpions while naked just to get in the class. Sucks if the GM doesn't have them in the area you're currently fighting and you're nearly at a level up.
There's also like 10+ that make you "Do X for Y time". Examples include spending 10 days buried alive. Or spend a month on the plane of fire. In a game with no formalised downtime for you to do that kind of silly stuff. Both are actual examples, not hyperbole, too.
Worse Mechanical offenders:
Dwarven Defender: 3 (extremely crappy) Feats, BAB 7 and an alignment. It's 10 levels long, too. So you gotta be a Fighter.
Magelord: Alignment, 3 skills, 4 Feats, 5th level Spells, Evasion Class Ability, 10, 000 Gold. It's 10 levels long, too. Which means you won't see the end pre-epic because you need 9 levels of Wizard and 2 (3?) levels of Rogue.
I think I'll stop there because I could literally go all night.