Every setting has dull parts and awesome parts.
I do agree that the Dragonlance setting was too strictly limited to the stories already told, and created the *perception* that everything cool had already been done.
Same with Middle-Earth, or other licensed settings (including Star Wars), IMO, and, as more and more Realms-shaking novels were written, and then canonically incorporated into the setting, the Realms quickly turned from a place for me to play to a place that I could look at, but not touch.
On the other hands, I think that Greyhawk could have benefitted greatly from the sort of loving attention to detail that Ed Greenwood lavished on the Realms (or that showed up in latter settings, like Al-Qadim or Kalamar). If I can't picture what an NPC looks like, sounds like and acts like, based on their culture, it is harder for me to consider them as remotely interesting, and, as much as I like Greyhawk, I couldn't tell you how anyone outside of the Scarlet Brotherhood dresses... (Yeah, that sort of flavor information is there to be found, but Ed Greenwood did a far better job of presenting that sort of thing, IMO, which is why the Realms always felt more alive, as a setting.)
Dark Sun, Ravenloft, Al-Qadim, Kara-Tur and yeah, even Eberron, seem to me sort of 'gimmick' settings, that showcase new rules and character options, with a setting constructed around those new options (even if the reverse was sometimes true, with rules being specifically constructed to better evoke a more 'arabian nights' or 'ninja vs. kaiju' feel). Such an artificially constructed nature made each of these settings feel more coherent than Greyhawk or the Realms, that sort of came together over time from may different places (more so in the case of the Realms, which quickly turned into Fibber McGee's closet, as a dozen other contributers strapped wildly disparate lands onto the groaning back of Ed Greenwood's quaint rustic little 'Forgotten' Realms).
On the downside, for the Realms, too much of anything can be overwhelming, and the Realms followed the Greyhawk tradition of having dozens upon dozens of gods, including whole pantheons devoted to particular geographic regions, or demihuman races. Dragonlance and Eberron have done away with that, while other settings, such as the Scarred Lands, originally presented only a small core of gods, but then (IMO) started down the road to whackiness by presenting demigods for the various other races that often seemed to be somewhat redundant (or even kinda setting-contrary, in the case of the Asaatthi demigod!).
Lots of options are good for players (and can be good for DMs, if they are willing to sort out the chaff and dispose of what they feel is clutter), but when each setting book has a three page section devoted to where members of *two dozen* different races fit into the Underdark or below the Shining Sea or whatever (and, all too often, introduce a few *new* ones in the back of the book), it's a bit out of control...
In short, a lack of theme, or tight creative control, leading to a scattershot setting, can be, if not dull, serve to make the setting increasingly bland and 'muddled.'