They pretty much laid it out, but here it is:
1.Your heroes are the heroes. As you read through the Worldbook, you'll quickly notice that... there's no Elminsters, Gandalfs, or Tensers. No Drizzts or Fahfrds. Your heroes are front and center, in a world where there's horrifying NOPE of one sort or another in any direction. Even the good and neutral kingdoms have something that makes them a bit dark or exciting. The tone is closer to Greyhawk or Lankhmar than, say, Warhammer - it's not grimdark.
2. It's huge and detailed. Only the Realms and possibly Mystara have more content. Kobold has published over 230 adventures of varying length, from four-page oneshots to campaign-length epics. The Worldbook is only matched by the 3e Realms FRCS - plus there's the entire Southlands book as well (and from time to time they hint that there will be an Eastlands book eventually too!)
3. It's the "official" unofficial setting of D&D. It was started at the TSR offices in the early 90s by Wolfgang Bauer and Jeff Grubb as their lunch game. Everyone from Ed Greenwood to Monte Cook has contributed to it along the way. It's D&D "The Director's Cut" - where they put their ideas that didn't fit in the other settings. As mentioned in the thread, it's willing to spend time on a lot of adult themes that WotC is more squeamish about - sexism, racism, slavery, sex, drugs and human sacrifice are very much part of this setting, without getting excessively graphic or disgusting.
4. Deep Magic. If you want a setting where you can cut loose with Monty Haul levels of magic treasure and have it make sense... this is the place. And given how much of it they publish... there's a lot. Plus, virtually any third party magic supplement thus fits like a glove somewhere in the campaign without unbalancing it. (Get Sandy Peterson's Cthulhu Mythos 5E- it mixes with Midgard like peanut butter and chocolate.)
5. You can go anywhere. Want a fantasy Egypt? Got it. Want to go a-viking? Got it. Want to fight dragons? Got it. Want to lead a mongol horde? Got it. Like the Realms or Mystara, it's a kitchen sink, but a better constructed one.
6. It's your setting. Kind of getting back to point #1, but... there are no novels. There's some measure of canon, but you're not going to have some fanboy getting in your face about it. In fact, the Worldbook expressedly encourages you to blow the whole place up in creative ways. THE WORLD TREE IS GONNA BURN.
7. Mixed-up mythology. While there are some entirely new deities and cultures, this is the setting where all the world's myths are reality, and rubbing shoulders with each other. Because most all the cultures and religions have some real world analog, it's a feast for folklore buffs and it's pretty easy to make naming conventions actually believable. When I say, "this country is renaissance-era Bohemia with clockwork magic" "this is late medieval Poland if it had been overrun with vampires" or "this is the early modern Holy Roman Empire with elf-blooded nobility", boom. Everything can be encapsulated this way. The Goddess Marena? "The Virgin Mary as imagined by vampires". Nothing that requires much explanation. It's the world you already know - twisted and turned up to 11.
So yeah... to me, this is the Uber-setting for people who want something kitchen sink and well supported like the Realms, but with a unique flavor all it's own - and where you can sandbox with impunity. Adventure everywhere!
