This world won't have monsters, at least in the sense that standard Dungeons and Dragons settings have monsters. It's definetly human-centric. There are no elves, dwarves, or any of the "standard" races. There are no dragons, beholders, orcs, goblins, or any "magical" monsters of that sort. It's human beings in various sorts of conflict and cooperation with other human beings for 98% of the setting. We humans are, for the most part, nasty enough to one another without there being anything nonhuman to cause conflict.
I did say 98%, though. As I think through the cultural and religious dimensions of the setting, there are certainly room for genii, spirits, or demons -- something otherworldly and supernatural. I'm leaning toward some variant of monotheism for the principal religions of the world, something loosely like early medieval Christianity and Islam. Within that framework, there are restless dead, nasty spirits, and, perhaps, direct servants of the deity or it's adversary. I have to be careful, though, because I don't want there to be a dominant cosmological, Manichean theme for the setting. It's not a Good vs. Evil sort of place (though that's not to say there aren't people in the setting who believe the world is all about Good vs. Evil).
The other bit is that there are certainly monstrous animals. Giant sharks prowl the waters. Carniverous apes lurk in the cliffs. Huge scorpions hide in the desert sands. Deep in the jungle, there may be dinosaurs! Plenty of animal nastyness waiting for the lost and unwary.
I also think it's telling that I immediately think of all these things, monsters or no, as potential antagonists of players, rather than as part of the setting. This is a Dungeons and Dragons holdover, no doubt. I am trying to move toward a sort of naturalism in how I conceive of the the plants and animals, strange or no, in the world.