The first is the emphasis; one is a swords and sorcery setting, and one is a high fantasy setting. This matters for some people, especially those who prefer a grittier, pulp feel that occasionally crosses into weirdness.
The second is that because Forgotten Realms has been ... um .... touched so much by the powers that be, it is both very appealing to some people, while not appealing at all to others. Sure, at different times TSR/WoTC has published stuff about GH since 1985 (something something Vecna, some more stuff about the Sea of Dust, an update to the timeline for 3e, etc.), but GH never became the kitchen sink setting. It never had to deal with multiple timelines, and retcons, and continual novelizations.
That's why they forced Kara Tur and Al Qadim and ... well, pretty much all of Greyhawk into FR (hey, look kids of the FR, it's Big Ben, Parliament, and Acererak, famous lich of the Forgotten Realms!). It's only mildly surprising that they haven't incorporated Eberron into it (the newest undiscovered continent of Toril ... Eberron!).
Plus all of the sunderings and plaguings and fracturings.
(I say that jokingly; it's not like people can't make fun of GH- look at the history of how Gygax filled in the map names sometime)
Now, there are some people (and many of them post here) who are totally into this! They love getting into the lore; digging into the rich history of FR that has been detailed through multiple sources; they enjoy trying to understand how these multiple sources interact and influence each other (how canon is an FR video game?); they love that FR has ready-made areas with different "flavors". Being able to draw on this rich and detailed history that is already fleshed out is not a bug, but a feature! And that is totally great for them.
But ... not everyone wants that. Some people want a classic swords and sorcery setting that hints of greater adventures, without all that lore. The idea that the "known world" is really just a small part of Oerth*, and you can fill in the rest. And GH scratches that itch.
*Speaking of funny names![/QUOTE]