I was just reading "Isle Of Dread" latetly, mostly because of all the awesome that is going on over at http://swordsandstitchery.blogspot.ca/. There is a small section within the module that basically gives a little break down of the Known World (Mystara). I'm not entirely sure if this is the first instance of Mystara, or a campaign world within D&D. I have to admit I quite like it! Through out this discovery process of OSR I keep finding these awesome things, that I didn't really now about before, or had just omitted at the time.
I grew up with 2nd edition, and it was very focused on Forgotten Realms. Yes I saw the ad's for Mystara at the time, but I wasn't buying these books for myself. As well I hadn't really formed an opinion.
For me, that little break down in Isle of dread is enough to go on, and get my creative juices flowing. Without all the extra awesome history you can find at - http://www.pandius.com/
There is actually a crazy amount of resources out there for Mystara. It's a world that keeps evolving and that is totally cool. But as I said, you can strip it back to what is in Isle of dread, and write if your way from there if you like.
There's a handy little Google Plus Community right here - https://plus.google.com/communities/112849302659541494005
Atlas of Mystara - https://plus.google.com/collection/Ymg_-
As you know, I've already fallen in love with Thunderrift, so it was only logical that I would dig a bit deeper.
My plan is to probably start running the Thunderrift adventures, and then hopefully move on to Isle of Dread. After that I'll have to do some research.