I feel a little bit differently than I think a lot of other people in the thread do. It really doesn't matter to me one bit the extent to which the world has a deeply realized history or a deeply realized anything, really, except to the extent that it directly affects a campaign that I'm going to be running or playing in. I get that it's possible to look at basically any piece of text written in any setting sourcebook ever and wring something out of it, but that's a very low bar. Some material is just more inspiring than other material. When I read a new setting sourcebook, I really want to be inspired by powerful ideas about the sort of campaigns that could only happen in that setting (or are especially perfect for that setting.) If There Are Some Kingdoms and it happens that Those Kingdoms Are In Conflict, it matters much less to me what the intricate histories of those kingdoms are as it does why that's a backdrop that I should choose for a campaign over any of the other campaign settings available or something that I just make up. This doesn't mean that the setting has to be a total wackyworld, but I don't need yet another arrangement of generic fantasy stuff without strong hooks.
Distilling my experience with different settings, I'd say that the single most important thing a new setting can have is brightly-colored ideas about the sort of things the player characters can be and the ways in which that affects their interactions with the setting. Clear hooks about how characters might be attached to the setting in interesting and new ways. Similarly, a setting with a smaller number of strong, focused ideas is better than a setting that's just sort of a collection of stuff that's kind of around. I can fill in the stuff that's just sort of kind of around on my own, as it matters.
The sort of things I'm looking for aren't mutually exclusive with things like pages of history that it's not obvious to me why I should care about that, but I feel like a lot of settings are presented as though that's the exciting part. The exciting part is inspiring ideas about what sort of person it'd be fun to be in a setting. Take Iron Kingdoms, for example. (I know it's not a d20 book, but it's a good example.) One of the things I dislike most about the book is that it has a tremendously inspiring character options section that makes me want to play in about twenty new campaigns every time I read it. It's absolutely packed with awesome ideas and on top of that is an amazing window into what the campaign world is like. Unfortunately, that section, including the facebreakingly incredible careers section, is buried halfway through the book behind a bunch of dramatically less inspiring geographical and historical cruft that's not the worst example of the form I've ever read, but is all but impossible to care about until you already have a pretty good idea of what the world is like (and even then, it's hard not to rapidly go into skim mode), and it does not do a good job of introducing the world.
I understand the impulse to present a setting like it's a real place, but in the end that is just not as useful to me as a setting book that realizes that it's being read by somebody who wants to be a player or a DM in a TTRPG adventure set in the setting.