I started playing D&D in 1986 with the Mentzer "Red Box" Basic Set and "Blue Box" Expert Set, and thus "The Known World" was the very first campaign setting that I was exposed to. Even after moving on to AD&D 1e rules, I still kept running games in The Known World, as it was near and dear to my heart and the Gazetteer series was fantastic. Even after I bought the 1e World of Greyhawk boxed set, I still prefered the Gaz series, as it felt more wondrous and fantastic, and less stodgy and conservative.
Eberron reminds me a lot of the D&D Known World (or "Mystara" as it eventually came to be known). Not in terms of thinly-disguised Earth cultures transplanted to D&D, but in terms of the feel of the setting. Mystara had a lot of pretty out-there ideas, and wasn't afraid to include them. When I read the Eberron books, it brings back fond memories of my days running games in The Known World -- The Principalities of Glantri, The Isle of Dread, Karameikos, the Republic of Darokin, the elven clans of Alfheim, The Savage Coast, Norwold... Reading Sharn: City of Towers constantly reminds me of Glantri City in Gaz 3, which is a good thing.
More than any other official D&D setting, Eberron really takes me back to that feeling that I had when I started running D&D games with the map in the Expert Set (and X10 Red Arrow, Black Shield) and the Gazetteer books. But it's a more refined, better-developed setting, without a lot of the silliness. The Known World has always been one of my favourite settings, but it did admittedly contain a lot of pretty silly elements. Eberron feels like a more grown-up version of The Known World that still maintains that sense of the wondrous and fantastic, without limiting itself to a very stereotypical view of what D&D is supposed to be.