I have a few ideas about how prominent aspects of the Kingdoms of Kalamar setting compare, though I'm sure other folks who have played in all the settings can come up with more.
Magic Level: Kalamar is not as dramatically high in magic as Eberron or the Forgotten Realms, and most inhabitants of the world aren’t exposed to magic on a routine basis. This lets the DM tailor the world to his or her own magic taste. It’s easy to sprinkle in more magic if that’s the way you like to play, but it can be more difficult to take magic out if an entire society or town is based on some magical element. Probably similar to Greyhawk in that respect, though to be honest I've never personally played in GH.
Power Level: The KoK setting is driven by the actions of ordinary men. The PCs are the exception, not the rule, and high-level NPCs over 13th level are rare. Like magic, it's easier for the DM to add power if he wants to.
Geography: The world was built from the bottom up. We considered plate tectonics, placed mountains based on that, set weather patters and bodies of water, defined migration patterns of early inhabitants and eventually developed the modern societies all in a logical progression. We can contrast this with Forgotten Realms or Greyhawk. My understanding of these settings is that they were based on D&D campaigns of Gary Gygax and Ed Greenwood. The players probably started somewhere and the world grew out from there as the DMs needed to fill in the details. This type of top down world is fine if that’s the way you want to play but it can lead to some logical inconsistencies. For example, Kalamar has six human races, each with their own language. Thus, each place that race populates should be named in the language of the people who live there. An exception to this would be if the place was conquered. The place name is often renamed by the victors.
I've glanced at the Eberron map, but that was a while ago and I don't remember how realistic it looks.
Hobgoblins: The hobgoblins of KoK are a major force with two empires, and available to PCs as full-blooded or half-hobgoblin characters.
Deities: There are different deities, naturally, although the appearance of a deity on Tellene is a much rarer event than in some other settings (FR springs to mind); they prefer to manipulate events through their clerics and followers. Their ultimate goal is to gain new followers and continue to influence current ones. A god’s power is directly related to the number of followers gained and the prevalence of the god’s spheres of influence. Therefore, the gods always strive to spread their power by gaining followers and clerics.
Kingdoms: Naturally, the kingdoms and such will be different. The KoK setting features elements common to many popular fantasy works, including J.R.R Tolkien’s Middle-Earth from “the Lord of the Rings,” Robert E. Howard’s Hyboria, from the Conan stories, and the land of Robert Jordan’s Wheel of Time, among others. Naturally, some will be common to FR, GH and Eberron as well.
The feel of the land depends mostly on what region the player characters are in. For example, the Reanaaria Bay region is standard fantasy common to most other settings, where the human and humanoid races trade among each other and the enemies are mostly bandits, pirates and monsters. Most other lands, however, do not share this view of life. In the Kingdom of Eldor far to the west of the Bay, humans are fiercely proud of their ancestry and strive to keep their bloodline pure, and visitors of other races are persecuted, while the military fights a skirmish war with the elves. In the Theocracy of Slen, to the north, the clerics of an evil god control all, and darkness lies across the land. The Isle of Svimohzia, too, is a study in contrasts. This ancient land of dark-skinned peoples holds cities of magnificent beauty, war-torn lands, and a great looming jungle that devours the foolhardy.