My opinions on them:
Greyhawk: The_Gneech got this one pretty right--nice straddling of S&S and high fantasy, has probably enough cohesion, yet still lots of DM wiggle-room. In fact, it's not so much wiggle-room as DMs are required to do a lot of work on their own; either that or hunt down products that have been out of print for decades. Silly names. The setting is also boring as all heck. There's nothing about Greyhawk to condemn it really (other than the Rose Estes novels) but by the same token, I can't think of anything about the setting to recommend it either, unless you just really like the legacy or the nostalgia; two things about which I care very little.
Forgotten Realms: I also agree with The-Gneech for the most part here; FR is big and silly, baroque and colorful. It's got some cool idea (Red Wizards, anyone?) but doesn't always utilize them in a good way. Also hampered by a long history of fanboys--heaven help your campaign if you've got one in your group while you're trying to run--and novels, most of which range from mediocre and forgettable to "time for a good ole fashioned book burning." Because the setting developed along lines explored in the books, that often led to some extremely awkward game products. More recent products have mostly corrected the most egregious of the 2e excesses, but to me, FR is still more about one of three things depending on the DM and players, 1) power-fantasy wish-fulfilment, 2) some cool stuff that can be stolen and used in a setting that isn't so cheesy, 3) folks who either remember original FR products, or otherwise tweak the tone of the setting somewhat.
Kingdoms of Kalamar: You either really like or really can't stand this setting, it seems. It's M.O. is realism within the confines of the D&D ruleset. It's for folks who are also hobbyists of anthropology, geology, climatology, linguistics, etc. and it has (relatively) rigorously applied concepts from those fields to the setting development. It's lower (or rarer, to be more accurate) magic than even Greyhawk, IMO--it's like Greyhawk, but less organically grown and with a greater eye for consistency. For those who don't like it; it's too dry and dull. I'm a big fan--for my traditional D&D needs, this is my setting of choice.
Eberron: This is also built with an eye towards consistency, but with a slant; incorporate the D&Disms more thoroughly while simultanously putting the "noir" or "pulp" filter all over everything. There's still lots of Eberron for DMs to explore (or create) although as the years roll on that'll probably eventually shrink. I really like the design philosophies and the tone of Eberron, and I like that it can accomodate all kinds of setting very well. It also not only has some great content to crib for other settings (even if you don't like warforged, the changelings and shifters are great additions to even uber-traditional fantasy campaigns) and lends itself fairly well to borrowings from other settings too (I'd love to put Red Wizards in there somewhere, although I haven't yet figured out where...)
Coming soon... I have to take a friend to the airport, but when I get back, I'll do a bonus analysis of some of the unofficial 3e settings that I have.
