What everyone else said.
Though I haven't read all the posts (and the below has no doubt been covered by much more talented people than me), the main thing is consistency and constant motion. Consider earth, always in motion that works in perfect synergy, wind effects water, water effects rocks and sand, bushfires lay waste to forests so that they regrow. Everything works in unison. If you have something that is a blight upon the world, then there needs to be a reason for it. Volcanoes, deserts, typhoons, earthquakes, they all happen because of a multitude of separate events that come together. Have nothing in your world for the sake of it.
On a similar note, watch the arbitrary naming, and try to avoid cliches. Apologies if this seems critical, but remember that place names evolve over centuries as wars are fought/ land is seized and so on. Consider Earth, not as a template, but as an example. There are very places that are named 'The [insert noun] of [insert noun/ adjective'. This is especially true when you consider your inhabitants and their histories. Consider the United States; New York and New England are obviously a result of British colonisation, while Las Vegas and Los Angeles have Spanish roots, while still others have French names, such as New Orleans. Again, this harks back to a living, constantly-in-motion world.
This has been mentioned above, but it's worth saying again: no country is inhabited by a single race of people. Even in a world where travel and communications might be perceived as medieval tech-wise, there are merchants, traders, mercenaries, invaders, scholars, adventurers, criminals, all of them travelling all the time. In our own histories there have been the crusades, East India Trading Company, gold rushes, prison colonies, wars, and even good old exploration that resulted in 'races' being spread to the four corners of the globe. Unless you have two neighbouring countries at war, people from both sides will be all over the place. Of course, there are exceptions (ignoring my earlier caution on cliches, you can have dwarves in their mountain mines, elves living in deep seclusion in a forest, etc), but as with everything there needs to a solid historical reason for every aspect of your world.
On being full on about maps and detail, this can lead to heartache. I have redrawn my maps so many times that I can't even remember what the world first looked like. For example, character X needs to get to place X before villain X kills character Y. Damn, Character X can't make it in time due to distance, immovable barriers, etc. Back to the drawing board on your map. By all means know what lies beyond the sea, or roughly where a mountain range is, but be wary of allowing those to restrict the story (though in some cases this can improve it- I have had a few beloved characters die because the protags couldn't get there in time).
Sorry, one last thing- watch the licensing on the mythology. While nobody 'owns' elves, dwarves and so on, pretty sure Blizzard owns 'murloc' and all that the name entails (I could be wrong, but I haven't heard of them anywhere other than WoW).