Forgotten Realms has always been, by design, a melting pot. An everything goes, let's have everything for everyone type of campaign. It's also quite high magic, high fantasy.
Greyhawk had more of an old school feel. So armies of orcs, not armies of undead lead by a lich king. Think LOTR versus WOW. Both fun in their own rights, but different.
Or at least that's my perspective. I'm sure other people can give you much more detail.
That makes sense on paper, but in practice -- or when describing it on the back of a book -- both settings sound like the same generic fantasy setting. Back in the 3e days, I knew people that thought Greyhawk was just a different region of Forgotten Realms.
In some ways, I think Dragonlance might be in the same boat, but it *is* easier to differentiate it by focusing on the more romantic or character-driven details. You'll still have a problem reconciling the reality that most players love Dragonlance because of the War of the Lance stories, but the settings always seem to default to the ages after that (ages that are much more generic).
I think with Greyhawk, they'd need to find a way to focus it on a theme of some sort (and marketing it as a more old-school setting just isn't going to cut it. Maybe Greyhawk could be 'dark and gritty fantasy setting' to contrast the 'high magic fantasy' of Forgotten Realms. I could even see elements of the old Birthright setting getting incorporated into a Greyhawk reboot.