There's a school of thought in the Old School movement that says that monsters generally should be unique things. I understand this impulse even though I don't subscribe to that school of thought - mainly because I love books of monsters. Lamentations of the Flame Princess and the recent "Psychedelic Adventures" series by Geoffrey McKinney have gone the furthest in seeing this through, although there are discussions to this effect by others. Seeing Better Than Any Man, the LotFP Free RPG Day release, has sort of cemented some of my thoughts on this.
One result of this is that there are, well, unique monsters. These are cool, because they're always new and different. Sometimes they follow themes, such as the animal-like creations in Geoffrey McKinney's Isle of the Unknown and Dungeon of the Unknown. (The former is usable as a virtual bestiary on its own.) Other times they're just truly bizarre things, like some of the creatures of Better Than Any Man. There are plenty of good routes to go here, and between Matt Finch and James Raggi's books for designing monsters, there are good resources to go any of them.
The second thing you see in these modules is a lot more human antagonists. It's quite easy, and indeed some people champion the idea, to have the entire cursus honorum of humanoids replaced by humans of various aspects and types. Bandits, brigands, cavemen, berserkers, and so on become more important, as do unique humans with class levels. It makes a world where some humans are the worst monsters of all. At the same time, I've found that parties are less likely to charge blindly into combat with humans instead of at least attempting parley, so it's a mixed bag overall.
The last thing we see is a lot of giant animals. Mutant animals, magic animals, animals that are extra vicious for one reason or another, insects, mammals, reptiles, arachnids - pretty much running the gamut. The tendency to do this is pretty universal among the modules I've reviewed, and it has interesting world-building implications. Why is the world full of giant/mutant/magic animals? Does it have hybrids like owlbears? Where do beast-men fit in all of this? The questions are interesting. One thing I'd like to see is fantasy fauna - that is, animals that are mundane but otherworldly - sort of a fantasy equivalent of the thoat, calot etc of Barsoom. It'd make these a bit more varied.
On the whole, I enjoy the modules that use this approach. They're fresh and different, and have useful ideas. But as a great lover of monster books I can't go along with the idea, even though I enjoy the fruits of their creations. The diversity in all this is really one of the best things about the Old School gaming movement today.