Tim Baker wrote:
Perhaps it would help if someone was able to summarize how Mystara, the Realms, and Greyhawk are different, ignoring the Hollow World for now (which is clearly one significant difference). Since I'm already familiar with Midgard, if there was a certain flavor that each of those D&D captured best, I might be able to align Midgard (or perhaps only portions of Midgard) to one more than the others.
I do not know well enough Midgard or Greyhawk to comment on them, but I do know Mystara and Forgotten Realms quite well.
The most important thing I would say for both world is that they are the most developed D&D world (FR mostly by canon, Mystara both by canon and fanon), and therefore so varied and diverse you can hardly summarize them.
Agathokles already pointed out some of the unique characteristics of Mystara.
I would say both have regions of very high magic (Alphatia, Halruaa, Netheril and Shade) and regions where technology is used (Savage Coast, Lantan) and several regions inspired by RW cultures, even if in Mystara almost every nation or culture is inspired by RW ones, while in the FR some are but the main region of Faerun is a generic medieval/renaissance area, even if the creator, Ed Greenwood, tried to some extent to give it an unique identity, at least in the names of places and people.