If they answer "Forgotten Realms", you should sit there, silently, glaring at them, waiting for them to explain why, unbidden. If they hesitate, they're gone. If they say, however, "Ha ha I was just kidding it's Greyhawk," then with skeptical caution they should be let in.
To start, when Gygax left TSR (or rather, pushed out of) Greyhawk kinda got into a weird place that got worst due to Forgotten Realms and DragonLance success, at that time TSR realized they were making money out of their novel lines and Greyhawk was pushed into getting the same treatment, however it never got the success of those two settings that should not be surprising considering TSR had no idea what the fuck they were doing.
As for Forgotten Realms, Ed Greenwood articles on Dragon were great but its success, especially the novel line, created problems and stuff was just dump on it, for example the Moonshae isles were just drop there despite they would likely been on Greyhawk but the worst offenders was the fucking novel line, basically they allow novels to interfere with the setting making the whole place having too many heroes all over the place that often had "chosen one" status (I am not even talking about just the Chosen of because of course everyone had to create their own Elminster) so players got the impression they werent needed because you had so many powerful people going around, granted it was NOTHING like DragonLance that became just "hey! remenber those books? wanna play DnD in there?" because they completely overshadowed the setting and it didnt help they keep having apocalyptic events on the novel line.
Forgotten Realms is a very decent setting
IF you know what material to use, basically the type of stuff Ed Greenwood wrote for Dragon and ONLY that, if you start looking at the "bigger picture" then decades of having crap thrown in makes the damn place a landfill and that is the problem with players because they have access to all that crap and can annoy you because of "lore" and this is why we have a DM in the first place, playing in Forgotten Realms means you have to learn how to say NO a lot unless you manage to find players that understand they arent in a "Living Campaign" (if that is still around) and dont start acting like assholes but you get then whatever the fuck you play if its printed or you think you dont get those with Greyhawk? There is always someone that managed to read every fucking novel and moment you get out of the lore they will call on it.
And this is why your own campaigns are the best but that involves
effort and you cannot just pull some printed campaign setting book and fill in the blanks, no you actually have to come up with your own shit ... Eberron was supposed to never advance and be static so I guess there is that but guess what it have? FUCKING NOVELS because of course it would ...