The big uniques of Greyhawk are things like:
Scarlet Brotherhood political behind the scenes manipulator evil monks.
Iuz - evil demonic demigod open ruler of an expanding empire.
Wastri - demigod of a human ethnic racial purity and hatred of demihumanoids but also a swamp and frog aspect and lives in the world.
Zagyg - ascended mage demigod of chaos magic.
First, I will quote myself as to
Why Greyhawk?
Greyhawk is D&D before people thought they needed to color within the lines. Greyhawk is D&D your way. It's the infinite multiverse and crashed spaceships, it's ninja nazi monks and demon-possessed emperors, it's endless black ice and dead civilizations blasted by colorless fire.
It's a mechanical bejeweled songbird from two millennia ago with powers that amaze and terrify, and an ancient computer designed by a long-ago Baron driven insane by his creation. It's a dark nameless god dreaming within a crystalline cyst, and demi-gods raised from the ranks of mere mortal adventurers.
It's a land that contains both the hospitable and brave free people of the Yeomanry who regularly elect their leaders from amongst their ranks, as well as the infamy of Vlek, the Stonefist, who acquired power through the slaughter of the Coltens Feodality under cover of negotiation.
I would add to your comment the following- in addition to the themes I outlined in the original post, and your excellent additions, I would say that the true strengths of Greyhawk are as follows:
1. If run as the Boxed Set/Folio/DMG (from what we understand) version, it truly is a setting that provides scaffolding and some hooks, but otherwise is canvas that every table will paint differently.
2. It has a lot of different "genre options" (you've mentioned many of them, such as fantasy Pirates and fantasy Vikings). It also has a middle-eastern tinged area, which may be a plus for some, in addition to the standard more "Western" tropes.
3. The politics and political systems are truly diverse! Greyhawk is notable not for the monarchies, but for the wild diversity of government types- rule by clergy (e.g., Almor), anarchic chiefs (e.g., Bandit Kingdoms), hereditary principalities (e.g., Bissel), an individual elected from the gentry (e.g., Dyvers), an individual elected by the people from the nobility (e.g., Gran March), an individual chosen by the oligarchs (e.g., City of Greyhawk), a freely elected individual (Highfolk- free town), a Magocracy of multiple wizards (Spindrift Isles) or a single wizard (Valley of the Mage), or a ... kind of Republican form of government formed of free citizens (Yeomanry), autocrats who seize power and rule with terror (VLEK!), Demigods (Iuz) ... etc. It's not just your standard, "Let's go meet the King" kinda place.
4. The gods are unique to Greyhawk. Whether you use the long prior list, or the shortened DMG list, you are getting some new gods. Plus, as I already wrote, the barriers between mortals and deities seems more porous in Greyhawk.
5. Greyhawk truly emphasizes that the present is not as advanced as the past. The artifacts and relics include machines and giant robots and mechanical songbirds, and, of course, the magical powers seem lesser than they were (RAIN OF COLORLESS FIRE!!!!). There is something fascinating about a fantasy setting where it's not just stasis, but actual decline.
6. Greyhawk is just the Flaeness, which is one part of the continent of Oerik. There is an entire rest of the world for tables to make their own.