Here's another one:
You have some of the best artists in the world. You have some deranged people working for you that can come up with amazingly wierd and wonderful monsters, animate them wonderfully and use Carmack's magic to make them drool and do wierd and wonderful things. Yet throughout the game 80% of the monsters are Imps.
Consider HL2. You have Combine, then Zombies, then Antlions, then more Combine, then elite Combine, then Gunships. You fight different things - helicopters, rollermines, etc.
FFS, Daikatana has more monster variety.
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We don't go to Ravenholm.