Okay, that's it. Give me a good sell on Shadow World. 
I'm gonna try...
Shadow World is a very unique and inspired setting, always expanding and regularly updated by the author (god bless you Terry), although not very rapidly since it is not his main occupation.
The planet Kulthea is quite vast with many continents to explore. The setting is very High fantasy, with magic everywhere, and signs of an ancient technological civilization. The planet is described as one of the Spacemaster setting so you can even mix fantasy with scifi if you like, but you don't have to if you don't like the idea. The timeline of the world is extremely long and detailed, with a sense of a very old world with many adventures ideas everywhere.
There are many races to choose from, each with very detailed cultures. The two main continents (Jaiman and Emer) are described in details with many realms.
For those who like mysteries, there are a lot of secret cults and undercover organizations (both good and evil). The Loremasters try to stop the evil plans of the Jerak Arenreth, the Dragonlords stand in the middle to protect their own interest, while the Navigators try to maintain their monopol concerning long distance travels using magic. In addition, the divine aspect is very strong with large pantheons and many religious cults everywhere.
On Kulthea you can find almost everything you could dream of to run awesome adventures: large cosmopolitans city-states or desert wastelands, coastal realms and big oceans, high mountains or deep underearth complexes and caves, secret fortresses and hidden cities, both very advanced cultures like the Elves of Namar Tol or primitive people like the small Kuluku of the rain forest, parallel universes with fearsome demons, etc.
All of this bathed in the Essaence, the magic flows into which the mages tap to cast spells, but that can also create destructive storms, open random portals to other dimensions, and change reality.
The Shadow World setting allows you to use ALL professions, ALL spell list of ALL magic realms, almost all creatures, and ALL weapons of the RM/RMSS/RMFRP system without restriction.
It simply is one of the most detailed, impressive and challenging High Fantasy setting I know, and running adventures in it is a real pleasure.
