One interesting thing is the fact that you can place all the WotC worlds (for D&D) on a planet the size of our Earth. Or rather the Continents. The result is a lot of land and not enough water, but it can be done.
So, this means that the planets of D&D are increasingly small. The largest planet happens to be Toril (or Sherem). The planet with the Gold core. Another thing that I like about Harn is that it's set on a planet that doesn't pretend to be super small, but big enough to support about 6-7 billion people if possible.
Planets are big objects/places though. I found this to be a problem with the D&D worlds. Not so much Kethira, though.
I believe Robin decided to go as earth-like as possible...
...so that Terran mechanical models (Climate, wind patterns, ocean currents, Plate Tectonics, etc.) could be applied. Land/Ocean ratio is approximately the same, etc, etc.
Yael is roughly the size of Luna (though with a more regular cycle to help keep track of moon phase/date). Other slight differences appear, but overall the similarity allows you to achieve a level of "familiar but different" that is the Hallmark of a truly great work.
The Ancient One has returned !!!
Which was awesome. When I
Which was awesome. When I build planets, I take care about ocean currents and so forth. Harn is on a living, breathing world. Earthquakes, volcanoes, all these things are active.
And I like how you can use other systems with it too, making Harn live in other ways. When I "bought" Harn (I actually got my copy for free) I loved how module it is. I can insert it into an existing campaign world without a problem, ignoring Kethira.