The princess rule: nothing that would appear on the walls on a 6 year old girl can be in FfH.
But even more than that we are really trying hard to create a unique setting for FfH. I know we steal from a lot of sources so it may not always seem like it is, but we are actually careful about what we borrow from. There is a reason we dip into Norse and Celtic mythology but not Greek or Egyptian (well we actually have two pretty major events from egyptian mythology but we have covered them up pretty well). There is a reason we delve into judeo-christian mystisim and lovecraft, but not dragonlance or hinduism.
It doesn't have much to do with what I like and dont like and I have stolen from all of those sources for different D&D games. It has to do with the underlying feel and themes of those sources, that they can be combined together in a way where it feels like a unified whole. You feel like the elves of Erebus could share a world with the cthulu inspired overlords, that a war between the demons and angels fits alongside dwarven golem makers. And that the background stories of all these pieces come from the same place.
Thats not to say that we couldn't justify any addition we wanted to make (and maybe in some cases we have). But we try to be critical to make sure the theme of new entries seems to match everything else. We dont want to recreate the same fantasy archetypes you see in every fantasy game (this is the elf guy, this is the paladin guy, this is the magic guy, etc) but to create new characters.
Of course we do use common fantasy characters, there is to much rich mindshare out there on what an elf is, what a vampire is, what a dragon is, to not use that to our advantage. But we try make them uniquely ours.
Heroes of Might and Magic was a great series, but its big failing to me was that it feels like the factions are generic fantasy constructs. The nature guys have their my little pony monsters with leprechauns and unicorns and the undead guys have their skeletons and liches, and the human guys have their peons and knights. But there was never any real relationship between the factions. They didn't feel like they shared the same world.
Lastly, most fantasy monsters have a theme inherent in their nature. Most come from mythology and fables and their role in those stories was to personify a positive or nagative characteristic. Ogres personify gluttony, se Sez has them covered in the blood of its last meal (the creature with the eyes in its hands in pans labyrinth is an ogre and shown seated at a banguet table). Unicorns symbolize purity. Not to say that we can't break that theme and justify another version but purity isnt a very FfH theme (which is about corruption and redemption, not perfection) so we don't have much use of the thematic aspects of a unicorn.
Those are my thoughts at least. I know the temptation is to grab all the best aspects of all our favorite fantasy sources, but I dont think that always works well.