Pros, Cons & Miscellaneous
Figured I'd go ahead and add my three cents to this discussion with my personal opinions about the mentioned settings
Kingdoms of Kalamar -
http://www.kenzerco.com/rpg/kalamar/
* Pros: Well thought-out, well-rounded world. Of all the official published game worlds, it's probably the most 'realistic' one in regards to history and geography, with enough parallels with our own world history to make things understandable. Plus, they've got Ed Greenwood working on their first regional sourcebook...and let's face it, when it comes to World Building, he's one of the best.
* Cons: The main book reads like a Text Book. Very dry and kind of monotonous, with so much information that just seems to melt together into a big blob. It also doesn't really 'stand out' there, to me... nothing really out-there that grabs my attenion as being 'different'. Also, unfortunately, besides the Core Book and quite a few adventures, there's just not enough source material quite yet.
* Misc: Plenty of support for the world, and with alot of die-hard fans that could give alot of advice on the world, yet it still lurks beneath the radar of most players, so you'll have the advantage of them not knowing everything there is to know about the world, or have them buying the products in bulk in one-up-manship that I've (at least) had the misfortune to have in the past with players who made more money than I did. They do have a fairly decent web-site, however, which helps DMs to get a good grip of the world.
Scarred Lands -
http://www.swordsorcery.com/
* Pros: It's new and edgy, different enough from the other major worlds (Greyhawk, Forgotten Realms, etc) to be interesting, while still maintaing a core of similarity that won't scare away new DMs and players. There's plenty of sourcebooks out, so you'll have more than enough material to work with, and it's got a dark enough edge to appeal to more 'adult' gamers. They have a pretty good website support, and just look at this MAP! (
http://www.swordsorcery.com/images/SCARmapfin.pdf)
* Cons: The popularity of their sourcebooks...since their monster manuals (ahem, Creature Collections) came out before anything else, I know <b>alot</b> of players who purchased them, drooling over templates they could try and convince their DMs to allow them to play. But, this is a relatively minor 'con', overall, and not all DMs are put off by this.
* Misc: Heck, they're owned by the same people who publish the hard copies of Monte Cook's Malhovic Press stuff... gotta be worth at least some bonus points, no?
Greyhawk -
* Pros: Next to the Forgotten Realms, this is probably the best supported game system out there - heck, even the Core Rulebooks are written with Greyhawk in mind. It has a very rich and detailed history and setting, you get new information every month from Dragon Magazine, at the least, and it is the forebearer for all other D&D Game Worlds. If you don't know it, you can find it, either on the Messageboards, Fan-Sites or a zillion other ways.
* Cons: The antiquity of the world...while also a Pro, it can be a Con, as some players (and DMs) think that it's just overdone...what's left of the story to tell? Many people like 'new and exotic' to 'tried and true', and it can majorly influence players reactions to hearing that it's yet 'another' Greyhawk Campaign.
* Misc: Of the three mentioned Game Worlds, it has the best support... there are still stories left to tell in the world, and I think it's got more than a few good years left in it

Besides, how can you go wrong with the home of Mordenkainen, Tenser, Bigby and all the familiar 'friends' made by reading the spells in the Player's Handbook?
Though I personally don't currently play in any of the above game worlds, either as a player or as a DM, each one has its own merits and flaws... basically, you just have to decide which one suits your own personal likes and dislikes the best... check out the websites, read some of the messageboards and go from there
Hope this helped a little bit!
Christopher