Just so you know that I don't think the Fiend Folio is full of manna from heaven, I'll address one of my areas of dissatisfaction.
I'm not sure what the hell is up with the Flind and the Lizard King. On the one hand, advanced members of humanoid species are a natural expansion of the base rules, and totally reasonable. On the other hand, how the hell do we get from gnolls and lizard men to these monsters.
Not that I didn't use them back in the day, but even then, there was a high level of WTF-ery in using them. The Flinds, for instance, are the badass cousins of gnolls. Tougher, stronger, and worshipped by gnolls. But unlike gnolls, flind tend to use clubs in close combat -- except for the few that use "flind bars", which are basically nunchaku that allow two attacks per round for only 1d4 damage and cause an opponent to save or be disarmed. Big, nunchuk-wielding gnolls. Yeah.
Lizard Kings, who have no resemblance to Jim Morrison, and cannot do anything, can however, do 5d4 or more damage with their tridents (though the are just normal tridents to everyone else.) Yes, not only are they bigger and tougher and more evil than standard lizard men, they are single-minded masters of the trident, even though most lizard men do not use tridents. And are, in fact, pretty primitive.
I used them because I thought as a teenager that lizard men were pretty cool, and needed to be more fearsome because, well, they were lizard men. And I figured lizard men should just be hard core. They're seven foot tall lizard guys! Come on! But now... well, I'm still scratching my head on the super-trident-skills.
Not to say that the concepts are all bad, but the execution was a little bit out of left field.