I like a good flavour to a campaign. And if it's a gumbo that's fine with me. If it's roast meat and potatoes that's fine with me. As long as it works.
I gotta agree: Greyhawk would strike me as pretty boring if published today in it's original format. Fortunately it's grown along the way. And although it is still generic western mediaeval setting 1.0 I think it's got the depth to handle a good variety of play styles and campaigns.
On the Ebberron note: never played it (bar one rather brief foray into one module about an airship, erm, The Golden Dragon? Something like that.) I think the setting is OK but would work much better in another, more flexible, system. Say GURPS or HERO. And you can never have enough bad guys. One group will focus on a few of the BBEGs presented, another group will focus on others. It's a 'broad appeal marketing strategy.'
In fact I'd say that any published setting is many mini-settings put together as a package of related things. Which means that campaign settings usually get one or two bad ideas in them: an inevitable result of them being so big in scope and frequently the result of multiple authors/editors. That's OK, a group can ignore those bits of a setting that don't sit well with it.
Which doesn't actually help with specifics for the OP. Um... Specifics...
Nothing specific comes to mind. It's art. There is no hard or fast rule for art. What works in one combination does not work in another combination. And which combinations work changes depending on who is interacting with the art.
A setting I
don't like is Sigil. This is one example of where the execution completely misses the intent. It advertises itself 'as crossing boundaries, pushing limits, anything that the imagination can desire.' It delivers 'you sit in the pub with uber-NPCs to keep you in line until it's time to walk down the road to Plane of the Week.' The Planes are just different rooms in a dungeon. It takes all the alien weirdness out of being in different realities.
One area I think many setting go wrong is with official campaign progressions. It's up to each and every group to play its OWN campaign, not conform to whatever the hell someone back at the Publisher thinks should happen. But again, I feel free to ignore those bits I don't like and only grab those bits I do. Something about the emphasis placed on this stuff does get my hackles up though...
cheers.