I'd have to agree that Mystara doesn't really have much of a hook or overall theme to the setting, other than that it never seemed to take itself too seriously.
The "Known World" will always have a place in my heart, but I do think that a lot of my fondness for it is nostalgia. I started playing D&D in 1986 with the (Mentzer) Basic Set, and the Expert Set with X1 introduced "the D&D world" to me. B6 The Veiled Society and B10 Night's Dark Terror developed the culture and flavour of The Grand Duchy of Karameikos far beyond what was in the Expert Set. X1 The Isle of Dread, X4 Master of the Desert Nomads, X5 Temple of Death, X6 Quagmire, and X9 The Savage Coast developed the lands to the west of the countries on the Expert Set map. We then returned to those core nations for X10 Red Arrow, Black Shield which expanded upon those countries again and brought them all together for a full-scale war. CM1 Test of the Warlords introduced the northern land of Norwold and the Thyatis/Alphatia conflict, which set the stage for the rest of the CM and M series modules. I've read in other threads that Greyhawk was defined by its adventures, but I think that's even more true of Mystara; it was just a map with a short paragraph for each country until the adventures expanded upon it.
Even after switching to AD&D (1e, though it had been a gradual process of bringing more and more AD&D stuff over to the BECM rules), I still played in what would be known as Mystara as it felt like home to me and at that point the Gazetteer series was being published. I had the '83 World of Greyhawk set, but it seemed kind of bland to me in comparison. I eventually left Mystara behind around '88 in favour of the "grey box" Forgotten Realms set, as I was starting to really focus on things like internal consistency, which was never a strong point of The Known World. I grew to dislike the Realms after a while, but I do still really like the original presentation in that first grey box, when it left a lot to the imagination and didn't feel over-crowded. And eventually, Forgotten Realms, Dragonlance, and Mystara (after the whole Gazetteer run) all felt too crowded to me, much like the Star Wars "Expanded Universe" became after hundreds of novels and comic books filled in gaps that I didn't think needed filling.
After the Forgotten Realms, the "core" nations of Mystara probably have the most detail published of any D&D campaign setting, which I suppose is a hook of sorts. I started a 3.5e game in Karameikos (which I still love as presented in B10 and GAZ1) a couple of years ago that never went beyond a session or two due to real-life stuff, but my sister was the only player who really got into the setting (since she grew up playing in that world as well). I've thought about re-imagining it for 4e, leaving out a lot of the stuff that I find silly or inconsistent, but I don't know it will ever really be appreciated by players who didn't first experience it with the Moldvay/Mentzer Basic and Expert rules in the 80s. The Grand Duchy of Karameikos, as presented in GAZ1, still makes a fantastic setting for a new campaign, but many of the lands around it don't really feel like they belong. I still like Glantri, Alfheim, Rockhome, and the Shires (more or less as they are presented in the Gazetteers), but too many of the other lands feel like unimaginatively placed Earth cultures dropped on the map without much thought as to how they interact with each other. I used to think that Greyhawk was bland, but I appreciate it more now; some of the lands of the Flanaess are obviously borrowed Earth cultures, but there is a lot more thought as to how these cultures developed and interacted with each other.
The Hollow World could be a hook, but I'd more or less moved on to the Realms by the time the Hollow World stuff was being published, and I never owned any of it. I rather dislike the GAZ style hex maps that many rave about though. The art style of the FR maps (as they got better each time it was re-published) were more to my liking.
