I definitely like Eberron. It's different without being too different (I could really care less about the IK style steampunk with magic), the rules modifications for it are reasonably well balanced, and don't change the core rules a huge amount, the density is just about perfect.
One person said that they thought the small number of nations might be a problem--that maybe there are too few. But as far as I can tell, it's just about perfect. Let me explain why I think that's true:
In the ECS, we have 16 distinct regions on the continent of Khorvaire, plus Aerenal, Argonessen, Sarlona, Xen'drik, and Frostfell. Of the 16 regions on Khorvaire, approximately ten are civilized. (I say approximately, because a lot of them are really pretty debatable. Two of them are *definitely* hostile (the Demon Wastes and the Mournland), four are probably not friendly to most adventuring parties (the Shadow Marches, Droaam, Darguun, and the Lhazaar Principalities), etc. etc. There's a lot of variety here--in climate, culture, what kinds of encounters are likely, and so on.
But, at the same time, it's broken up into nicely digestible chunks. The recent book Five Nations covers the human nations (plus the Mournland, formerly a human nation). That's a good third of the areas on Khorvaire, and about the same landmass--and covers most of the range of terrain and culture. (Okay, no monstrous cultures here yet, and no desert.) More importantly, by having a single book covering just the Five Nations, there's plenty of room to talk about several power groups in *great detail* in each nation.
Basically, instead of having a zillion zillion different little nations each with its own agenda, we're getting less nations, and great detail on several organizations in each, each with different goals. That might not be everybody's cup of tea, but I like it quite a bit: it means that you can pick one nation and deal with the adventures and organizations local to that region, and then start to tie the organizations there into their continent-spanning counterparts.
And, most importantly, we're getting not just stat blocks for rulers, but goals and ideals, and the same for their opposition (both visible and covert).
Oh--and of course, aside from nations, we also have the dragonmarked houses, which are practically nations on their own (though not localized.)
Anyway, I think they've hit just about the perfect detail level with this setting. FRCS and GH are both great huge settings--but they're both so big that it's pretty much impossible to have both a detailed view of the individuals involved *and* a coherent global view at the same time. In Eberron, you can see how an invasion of monsters out of Droaam into Breland is going to effect everybody on the continent. Not immediately, not necessarily directly, but with any big thing like that, you start thinking about how the neighboring nations might want to grab some land (and why, and how, and under what pretense), how trade will be impacted, what secret evil organizations might do with the opportunity, etc. Suddenly, everything is in motion.
And that, I think, is something that most settings lack. That feeling of dynamism--how things will move if something changes. If things get too big, you start getting disconnects, so that things only matter locally. (Except when irritating do-gooders like Elminster exist to stitch the world together.) I mean--imagine Halruaa was blasted off the face of Faerun...
Who all would even notice?
It's not perfect, it's true. But it's the best setting I've seen in a long long time.
