Kobold Avenger
Legend
Only when compared to the standard fantasy worlds campaign settings, as it could be argued that Dark Sun, Ravenloft and Planescape have more of either grittiness or moral ambiguity. But in terms of the standard fantasy settings that may be true.Greyhawk is grittier - less forgiving and far more morally ambiguous.
It failed in terms of trying to establish itself as the default setting of 3e. The iconic characters like Tordek, Hennet, Mialee, Krusk and so on were intended to be people from Oerth not Toril, and the 3e PHB itself used Greyhawk deities like Wee Jas, Kord and Saint Cuthbert. But despite being the official default setting of 3e, it was barely developed in 3e beyond the Living Greyhawk stuff and whatever the short-lived revision of the miniature game Chainmail added to Oerth.Greyhawk did not 'fail' - it was deliberately dropped for the kitchen sink world of FR for commercial reasons.
And a lot of this comes down to the edition played in and the playstyle of the group rather than the campaign setting itself.Of course grittier rules make settings more distinct if applied to them..! 5th Edition especially is pretty easy to survive and prosper in due to HP rules, magical healing and recovery proliferation and the all-too-easy CR balance.
The problem in differentiating them is that they all are worlds with many of the standard fantasy tropes that focus in places that are vaguely late-medieval/early-Renaissance Western European (yes I know FR has Calimshan and GH has the Baklunish, for the often ignored Middle Eastern parts). The differences between FR and GH are smaller compared to the differences between Forgotten Realms and Eberron, or the differences between Greyhawk and Ravenloft. And that's what makes them harder to differentiate to non-hardcore fans.Now maybe all six of us are blowing smoke, or maybe the 120 years plus tabletop rpg experience round the table is able to see how different Greyhawk is to FR without the majority of them being 'hardcore fans' of the gameworld.
I am sure Mearls and crew will do the rather obvious differences between the two gameworlds justice. Those who haven't come across it before due to the saturation of FR through the system since 3rd Ed. will be able to make up their minds themselves.
I for one would predict they will have no problem differentiating them.