NOTE: This is an excerpt from the Developers Guide to the MRK which I was working on this morning. It's still a bit rough, but I'd love to hear what you think.
Here is where tabletop RPGs are different from all other forms of entertainment - books, music, movies, tv, video games, etc. - in creating these other forms of entertainment is a process which moves from a point of inspiration to a finished product, a state that must be reached before anyone can enjoy it.
With a tabletop RPG you go from a point of inspiration to a point of competency with “finished” left far off on the horizon. This is not lazy or wrong. It's a good because the play of the game, from beginning to end, session to session, campaign after campaign, makes up that other half of creation.
The whole traditional pre-game setup. The rolling of dice for abilities. The choosing of race, classes and equipment. All of that is like an artist setting up an easel, assembling ones paints and sketching out an idea of what to paint. It is not until the game gets rolling that actual paint hits the canvas and the strokes are made which will flush out a character and give it life. If you bring to the table a character who is already fully described, illustrated and advanced in level, a character who is 99% finished? That character might give you 1% play, if you're lucky. Detail adds weight and weight makes a character unwieldy, hard to shoehorn into a game it did not organically grow out of. If the character is a delicate flower of dependencies and multiple conditions then it can only go downhill as the action of a game breaks it to pieces.
As a Game Master, the world is your character. While there is something to be said about the fun of reading source books and studying the complete and fully flushed-out worlds of others as if they were vacation destinations, these same worlds can be absolute hell to game with. Remember, the players make the story and your job is to react to what they do. With a pre-fabricated setting, especially one that your players already know, your options are limited. Granted the MRK gives you the final word on everything, but that is not going to stop the bad feelings from flowing when some player tells you with definite authority that in the year 356 AC Lord Typhus defeated the Hoolerans at the battle of Crisis Ridge and although the Hooleran were obliterated under Lord Typhus's might their castle was not demolished and still stands to this day, supposedly with a dungeon chocked full of golden goodies to be plundered.
Because you haven't read that section of the source book, and had no intentions of going there tonight. You will have to tell them that – no - they cannot go to the castle of the Hoolerans. Because of the MRK they will have to agree with your decree, but quietly they will sit there and simmer and psychically sting you with accusations of rail-roading the game. For you it would have been better if Lord Typhus never existed.
If you create your own world then you only have to answer for what your players have already encountered. In this case the story lawyer can go from being your worst enemy to your best friend, helping remind you of what the party knows as opposed to telling you what you should have already known.
Creating your own world also brings back to adventure gaming something that has been lost over the years of splat books and source tomes (or is that tombs?) which is the thrill of exploration. For the game world you may have a blank canvas in your head, but the players don't know that. As player characters they assume there is something out there worth seeing which they haven't encountered yet. And there is! You just haven't yet encountered it either. Which is why it is not the worst thing in the world for these games to move at a less than speedy pace and with sessions which are days apart. While at work, or cooking dinner, or mowing the lawn - ideas will trickle in. This is yet another reason why the MRK has a website with a “Scribble Page” for quickly jotting down ideas without having to worry about making them fit.
Your friends will do this too, which is why the MRK has sharing and gifting. It's not just a matter of being friendly, it's an easy way for players to say, “hey the game needs this, I've put in my two-cents can you clean it up and make it work?”
World creation seems like a big task, but ultimately it is a group effort without a deadline. It is something you do while playing in the world. When done right, it becomes a part of the game and doesn't seem like work at all.