This seems like an interesting thread with a lot of great ideas flowing. Regarding Max's comment, (and not wanting to sound too academic) I think the question of fantasy v realism is the biggest philosophical question for people who make fantasy maps/worlds. Now, plugging my own comments, I started a post called Tips for Worldbuilders where I attempt to address some of these issues. I personally don't like the 'that's magic' approach - it just seems kinda lazy.

The title of the thread is about avoiding cliche. Again this is my personal opinion but I think the key to avoiding cliches is through developing the cultural traits of a people. As I suggest in my thread, if you start with one cultural thing, and then you slowly add to it and evolve it, you will eventually have a fairly 3-dimensional culture that feels complete and is much more original. I think that if you do that you can have a nation that is essentially Aztec (or whatever you're going for) but which is also unique, and you might find it takes you in paths you don't expect.

I also think it is absolutely possible to create a world different from ours without resorting to cliches like magic or familiar cultures. In realistic worldbuilding you have to stick within the parameters of plausibility (that's the fun of it), but that doesn't mean you cannot invent things. For example, perhaps smelting and smithing are difficult in an arctic climate; why not make something up that can be a substitute. Perhaps there are fossilized trees which, when sharpened up (using stone tools) become as hard and sharp as steel. No magic involved, solves the problem, and it is completely plausible (since it is a fictional place it is impossible to be 100% realistic, so what you're aiming for is plausibility and enough detail to cover up the mistakes). In addition, you have a spring-board for a culture: these fossilized trees are obviously important to the people, so perhaps they have a ritual for retrieving it, or somehow it is associated with their mythology etc.

My main point is that you have to be inventive in worldbuilding, and it has to be as plausible as you are satisfied with, but it can be done. There is no reason to accept cliches. It is probably always possible to invent a way around these problems. Even if you have to stay within the parameters that Max mentioned, the invention of a plant/animal/mineral/phenomena/ideology is perfectly acceptable, provided it is plausible. I'm interested in this topic, and, as time permits, I'll be watching to see where it goes. Hope my thoughts are helpful.