For hundreds of years colonist from Earth have lived on Kerren. They came to Karren to live a simplier peaceful life. A pity that dream never materialized.
Kerren is a world in an age of Saurians. Giant megafauna live among huge trees. Smaller saurians and giant insects live in their shadow. Humans live in Clusters, walled cities usually along cliffs to protect themselves.
Kerren is a world of little metal. The colonists expected resources to maintain a minimal level of technology. Those resources did not exist or are unobtainable given their tools. They adapted to the materials they had.
In short their life was simplier, but it was not peaceful.
Zhan are extradimensonal blobs that fall from rifts caused by electrical storms. Zhan infect living things, twisting them into rampaging monsters. If Kerren was not dangerous enough, now "things" were about.
However, the Colonists found allies on Kerren. The Dragons. Bonding with this alien species, The Ryders defend the world. They ride their dragons, destroying zhan as they fall before they can contaminate the world.
Kerren is like Pern, like every DnD world is like Tolkein... basically the same components, but packaged and presented a new way. If you are going to have dragons that are not stabled like horses, you are going to have something like Pern. Those comparisons are going to be unavoidable in the mind of anyone who has read Pern, but the same could be said about any Western Fantasy world and Tolkein.
It needs to be presented as a Swords and Planet world, one where you get elements of low tech and fantasy (Spears, Dragons, etc) and Science Fiction (Colonial StarFlight, Some technology that survives, and science explaining all the magikal elements (warp, Zhan, etc). The world has more limits on it (Dragon teleporting, Dragon semi-intelligence, zhan monsters, etc) that increase the drama and provide gamers something to do. The elements presented later in the thread are some of the things which make the world more unique (building, food, animals, and the Encyclopedia Kerrenian entries). In addition to all the tangible differences, any game pack created from this work needs campaign advice. The first one is the way to weave the characters together, in a way moreso than "you are all part of the same wing". The style of the game needs to promote soap opera punctuated monster fighting (given the scope of the monsters... if kicking monster butt is all you do, it will become boring soon... besides if you are doing your job, there will be no monsters). So you need style directions on how to present that kind of game. Lastly, I will need to provide megaplot lines for the GM, such as A Prime Warden situation, and a few others. Why is this not Pern.
The entire article is located at http://www.randomhouse.com/delrey/pern/amcc/article.html
The key quotes are I do have to be ruthless with those who want to write--or play in--Pern. Although I allow a fair amount of latitude (there are plenty of directions that don't violate my sense of what is possible, or permissible, on Pern), sometimes the imagination of those fascinated by Pern exceeds the bounds of the common sense that dictates what may, and may not, happen to the Weyrs, Holds, Halls...and, of course, the dragons. so I've occasionally had to police online games, and turn down ideas for MOOs, MUSHes, and IPGs. I have also had to staunchly turn down offers of ideas for stories. My works must be original to me, and I've no lack of ideas of my own to explore.
In short. Pern is hers. Even if I applied for a license, it still has to be hers. I totally respect that. If I had the Pern license, I think I could stay within the spirit, but I would be loathe to think I might stray beyond it.
Yet, Pern, as a roleplay environment, has some issues that need to be addressed. If I addressed those issues, I would stray beyond cannon. Thus, Kerren... a world that has similarities to Pern, yet is very, very different. That is why not Pern.