Midnight, Wheel of Time, and Arcana Unearthed are (or will be) all great, but they all diverge pretty far from the core D&D rules. They all use only a few classes from D&D, if any, and completely replace the magic system. I mention this because you said the following:
I would like these campaing settings to be easily played out of the box using the core D20 D&D rules and I would be discouraged to try any settings that contained house rules or innovations that stray too far from the core D20 D&D system.
Of all the settings recommended so far, these three all take a similar approach to heavily modifying the rules to better capture the feel of the world. How important, really, is this criteria to you? Perhaps you should reconsider, as it would open up more great options.
Anyway, here's how Kalamar relates to your needs, as I think it is a close fit just based from what you say:
I am looking for new campaign settings that offer informative and diverse sourcebooks, accesories, and adventures.
I would like settings that have a good selection of accessories, sourcebooks, and adventure modules, whether in print or in electronic format, from publishing companies that are committed to continue supporting their settings with future products. Price is not an issue.
Kalamar has very good support, as do many of the other settings mentioned. I think that Scarred Lands and Kalamar probably have the most supplemental material available out of those mentioned so far, with Scarred Lands edging ahead here for add-on rules material and Kalamar leading for module support.
This is where Iron Kingdoms kind of falls flat. Unfortunately the publishers of this setting are very, very, very, very slow at getting stuff out. The main setting book isn't even out yet after several years of quasi-vaporware status. It's a shame, because this setting has a lot of great potential.
I am looking primarily for new lands, races, peoples, cultures, and civilizations to explore as opposed to new rules/features like classes, skills, monsters, feats, spells etc. Although, if these campaign settings provide such new rules/features then I would consider them gravy and an extra incentive to try them out.
The Kalamar Campaign Setting has almost no rules in it at all. It puts a lot of emphasis on the various cultures and civilizations of the world and I think they really come alive well. If you want rules, you can get the Kalamar Player's Guide which has both more setting content, such as religious canons, and also rules material.
I would like these campaing settings to be easily played out of the box using the core D20 D&D rules and I would be discouraged to try any settings that contained house rules or innovations that stray too far from the core D20 D&D system.
I would venture to say that only Greyhawk is easier to play with the straight core rules than Kalamar, but Greyhawk isn't really supported very well. As mentioned before, the Kalamar setting book is playable straight up with no extra rules at all -- you could even run it easily in a different game system if you wanted (several people do exactly that).
I am mostly interested in European, Middle-Eartern, Asian, Native-American like campaing settings, with human/demi-human populations.
The six human races have similarities to various real world cultures, but are all unique and different so that you can't pigeon-hole them easily into real world counterparts. Of the cultural groups you mention, they are all represented except for Asian although there are a few rumors of another distant land across the ocean, so you could splice in Rokugan, Kara-Tur, or similar as a foreign land. One potential issue if you really like demi-humans is that demi-humans are pretty isolated in Kalamar. While all of the demi-humans are there and have their impact, humans are definitely in control of the world and take center stage.
I am looking for medieval-adventure-fantasy-warfare-political type settings appropiate for the D&D game and not for modern-science-spy-miniature-horror type settings.
One of Kalamar's greatest strengths is it's political depth. There is a delicate power balance and all kinds of intrigue happening. The whole world is kind of like Europe right before World War I, just waiting for a spark to set it off.
In many settings, politics are oversimplified by too much of a reliance on D&D's alignment system. I think that Forgotten Realms is an example of this. You know who most of the "good guys" and "bad guys" are. In Kalamar it's a lot more ambiguous, like in the real world. Nations struggle over conflicting interests, failed diplomacy, and to gain political and economic power. They don't usually oppose each other just because one of them is evil and one of them is good. Alignment is actually pretty much irrelevant at the political level.
Although I have not bought them or read them, by the number of books available for them in my local hobbie shop, I see that the Kingdom of Kalamar and the Scarred Lands campaing settings seem to have good support and diverse/informative products; so I am leaning towards trying them out. What are your thoughts on these settings and what others would you suggests and how any of them differ from the Forgotten Realms?
Scarred Lands has a strong theme of gods intervening in affairs and great powerful magic shaping history. Kalamar takes a more low magic approach, with the primary things that shape history being more pragmatic matters, like economics, warfare, and politics. The gods of Kalamar are more likely to work through their agents than directly intervene (although there are definitely exceptions). One setting is shaped by magic, the other uses magic more as a seasoning.
You should just pick up the books and flip through them and see if you can get a feel for what the two worlds are like. In the end I think it will just be the one that reaches out, grabs you, and feels right that works best.