The big problem with Isle of the Unknown for me was the monsters. As the Pundit puts it:
Every single creature is different, and usually a pastiche of various animals plus some weird quality. For example, a cat with metallic fur, immune to all mental attacks and ordinary weapons; it can see the invisible and has poisoned fangs. Or a bipedal frog the size of a man, who can fly and is immune to surprise, and has a slime spit. Or a bipedal skunk with bat-wings; where slaying it means the killer will later be pursued by its sire, who is a giant bat-winged skunk.
So there's no rhyme or reason to it at all; no tribes, no reason for the monsters to be there, nothing. Its a menagerie of crap, and I'm sure its meant to be "weird fantasy" but I'd put it closer to "stupid fantasy". The monsters serve no purpose, make no sense, in many cases what they do isn't even predictable (nor unpredictable in a good way; they just do things you wouldn't ever be able to expect for no reason at all).
Or as a different reviewer put it a while back:
Although occasionally spiced with some interesting abilities, [the monsters] really are giant pigeons all the way down: Pick a random animal. Make it bigger than normal. Randomly determine the number of limbs it possesses. Now, randomly combine it with another animal; light it on fire; have it ooze pus; or give it a random spell-like ability. Ta-Da! You’ve re-created the vast majority of the monsters in this book.
My aim isn't to further put the boot into somebody else's creative efforts: It's more to observe that creating totally new and unique monsters is really hard. Why? Human beings have been creating monsters for about 200,000 years, and that means the monsters that still exist today in the human imagination are beings which have survived 200,000 years of mimetic evolution. These motherfuckers have been honed for generation after generation, out-competing all the other mythical beings that have been dreamed up by children, parents, grand-parents, witch-doctors and story-tellers, and which have subsequently fallen by the wayside and been lost in the mists of time. They are core archetypes (the dragon, the succubus, the sexual bloodsucker, the man-beast, the faerie, the people who live underground digging up treasure, the beautiful magical bird) which, over millennia, have been carefully perfected by unconscious selective processes: the more a new variation scares, amazes and interests us, the more likely it is to be passed down.
Monsters, in other words, are like roses, dogs, and cattle: they've evolved through unnatural selection. We have bred out of them everything boring and unappealing through aeons of repetitive story-telling showing tiny graduations of improvement. They're the peacock's tail: they are that way because we, like the peahen choosing the peacock with the most impressive tail, carefully pick out the monsters we find scary and keep them going, and cast the others aside.
It's tough to compete with that. A few rare exceptions - creatures created by RPG bestiarists or fantasy authors - suggest themselves: the beholder, the owl-bear, the displacer beast, the bulette, the mind-flayer. But of those, I think only the mind-flayer truly comes close to competing with the resonance which a vampire, a werewolf, a succubus, a phoenix, or a giant has in our psyche. Those things have been around forever because they work. Randomly throwing together a mixture of animals and abilities simply can't compete.