Well, my take on the various settings, such as it is, goes something like this:
Greyhawk -- The most "old-schoolish" of the bunch, for obvious reasons, Greyhawk is loose enough that you can put pretty much whatever you want into it, but coherent enough that you can make it a believable setting. There is a lot of history, from the Invasion of Geoff to the rise and fall (and rise again) of the Temple of Elemental Evil, providing you with lots of potential adventure seeds. There are a small number of very powerful figures (e.g., Mordenkainen, Iuz, Iggwilv), but for the most part the players aren't likely to encounter them until they're a comparable level of power themselves. It does a nice job of straddling the "sword-and-sorcery" vs. "high fantasy" feel, but also has a lot of unfortunate place names, such as "Verbobonc."
Forgotten Realms -- Originally a slightly-more-verbose version of Greyhawk, FR has evolved over the years into a more baroque, high-magic kind of setting. More colorful and less grim than Greyhawk, FR can sometimes suffer from a kind of "Disneyland" feel, with beholders running the thieves' guild and ogres becoming bards. (To be fair, Greyhawk does occasionally have a similar tendency to wander into the bizarre, but there it's more of a "fell through a hole in the hedge" rarity, rather than an accepted facet of daily life.) FR has a wealth of detail that Greyhawk does not, which can be good or bad depending on your own personal tastes.
Eberron -- All I've seen of this is the setting book (of which I've read about half) and a few adventures, so all I can give you is my impression. Eberron is more 'contemporary' kind of setting, sort of "Miyazaki Meets
D&D," which of course leads some people to cry "It's too anime!" I personally see this as its main strength, rather than a weakness ... it makes Eberron a vital and exciting "new world" of gaming, while still building on the foundations of previous efforts. It also has echoes of '30s serials: besides the obvious "Indiana Jones" influence, when I see Sharn and airships, I think of Coruscant, which in turn makes me think of
Flash Gordon. Eberron would not be well-suited to run an "Against the Dark Lord" kind of campaign, but would do very well for "High Road to China." It's not as trippy-weird as Greyhawk, or as intricate-and-rich as FR; it's more about two-fisted rollicking action.
I only have passing knowledge of Kalamar, Krynn, Dark Sun, Ravenloft, etc., so I can't really address those. I could talk about Hyboria or Middle-Earth if you'd like...
-The Gneech
PS: These are merely my own descriptions of the settings as I see them. YMMV