I am actually facing this too: I handled it by making references so the reader goes "What?" and doing mysterious things so the reader knows that he's hiding something and may not be human, but doesn't know what.

Still fleshing out the details, though.

I'm facing the same problem. Mostly dealing with how to work with all the different races in my world, since I've developed each of them to a fair extent, but now am unsure how to bring it into the book, since I already have some of the races in there, but mostly as minor characters. So since it isn't a huge part of my book, I'm facing trouble figuring out how to give the readers enough knowledge that they appreciate them as much as I do.

It seems like a fair number of fantasy authors try to work around this problem by just having the ignorant character who doesn't know anything about anything and thus needs others to explain everything to him (and the reader.) Ignorant characters don't always fit in the stories, though

You might have to give up on letting the readers appreciate the races as much as you do. I mean, you're the author; you're always going to know the little details no one else does because they just didn't fit in the story. If you can bring out some of what makes your races worthy of appreciation through the minor characters, though...I dunno. That kind of thing is tough to figure out, hence my starting the thread in the first place.

Slipping info in without "info dumping" is a difficult issue.

Honestly, though, you sometimes have to info dump a bit, and not all info dumping is bad if handled correctly. Sometimes, starting off a chapter with a paragraph or two of background is not a bad thing, particularly when if pertains to what the reader is going to encounter next.

The one thing that drives me nuts is when authors try and slip too much into characters's conversations. It just makes it sound awkward and unnatural.

This works for when you have new characters, but what about when you're on a world where all of the races and cultures have interacted for centuries and don't need any explanation between themselves? The reader will know nothing between them all, so how would one work through that?

Other times, you have to give it time for the reader to learn everything. If you work in enough bits and pieces and hints, they'll figure out what they need to know. This sort of thing gives a book a bit of a flavor of a translated book, like Dostoyevsky's books or 'Heidi'. They give little to no explanation of the cultures they are in since they were written for those cultures – and yet people still enjoy the English translations immensely.