From my previous post I explained the origins of my interest in sandbox fantasy. But not why I was so obsessive about these details.
I was never much of a player of AD&D. Pretty much from the start I was a DM. I liked being a DM not for the power it gave me over players but because I just loved watching players explore the world I laid out in front of them.
As a player I was always too impatient. Always wanting to poke into the dark corners of a setting. Even today in MMORPGs I will level up a little and go off exploring. Granted as a DM I knew where everything was, but it was just as fun for me to watch players discovering stuff for themselves.
I wanted them to discover stuff not only in the dungeon but in the wilderness as well. This was main reason that Keep on the Borderland became my traditional starting module over Hommlet. Keep on the Borderlands had a area map. Having the player explore the wilderness was why it was important for the white areas to be filled in, that why the empty 30 mile hexes of Greyhawk bothered me, and that why the Wilderlands were one of the greatest gaming products I bought.
However I quickly realized that just because I was interested in it doesn't mean everybody else was. I had to learn how to make it interesting for people to play at my table.
The first trick I stumbled on to make this interesting was allowing my players to make lasting changes to the Wilderlands. Some DMs at that time got really bent out of shape if you "screwed" up their campaign world. The climax of my very first Wilderlands campaign involved the players carving out kingdoms in the Valley of the Ancients.
I read enough history at that point to know that in order to make a kingdom a success you have to do A, B, C, D, and E. The specifics differed but basically involved lining up resources, allies, and retainers. Of course we were playing AD&D so this meant that there were complications and that meant adventuring to resolve them.
In my first wilderlands campaign I ran a series of adventures for my players starting at 10th level. By the end they were 18th level. After this most of them had successfully established their kingdom.
The unexpected side-effect it solved the problem of balancing high level play. Large number of gold, and magic-items, doesn't really matter when you are struggling to build your realm. You can go slaughter hundreds of your enemies and the problems of ruling are still there at the end of the day.
The another benefit came in the next campaign when the new batch of characters had to deal with the new kingdoms that were established. It was then I discovered that the players actually respected and feared these kingdoms. Because all the defenses and tricks they used to establish their realms can now be turned against them.
By the time I graduated High School I had run a half dozen campaigns in the Wilderlands using the result of the earlier campaigns as background for the later ones.
So for me the most important points in running a Sandbox Fantasy are
1) Players should be allowed to have a meaningful impact on the setting both large and small.
2) The results of past campaigns serve as background for the current campaign.
3) The focus on establishing their legacy solves many of the problems of high level play.