The vast majority of games are run at the lower levels, where your PC's do not have much power and ability. As the DM, your job is certainly much easier. You have much more control over where they go, what their options are, and what sort of enemies to throw at them. Things are simpler, more controllable, and then that's why most games are run at this level. However, a High Level campaign is the opposite. All of a sudden your PC's have access to high level magic, and are able to do almost whatever they like. How do you handle this? How do you run a High Level Campaign without giving yourself a headache?
The trick lies in how you think about your job as DM. You are there to present the PC's with a plot for them to follow, as well as problems for them to solve. That has not changed. What has changed is how much control you have over their actions. To illustrate this, think about the classic dungeon: a series of rooms connected by tunnels, containing dangers. In a way, it functions as a flow chart, guiding the low level PC's from encounter to encounter. They have to follow the corridors, have to go through certain rooms. It stands as a symbol for low level games in general. As the DM, you have control.
Where this changes is when your PC's acquire the ability to break free of your flow chart. Teleportation is an excellent example of this. Suddenly they can jump from room to room, or escape the dungeon altogether back to the surface. As the DM, you have to rethink the flow chart, and realize that you can no longer control their movement. In effect, you have to erase the corridors from your dungeon, the arrows connecting the boxes on your chart. Now you still have to create your encounter, but they can be accessed and left at any time.
Furthermore, as your PC's grow more powerful, the nature of combat changes. Combat becomes increasingly easy or increasingly provocated and difficult. Your PC's will either mop the floor with your bad guys, or get stuck in for several hours worth of fighting. So while combat is still important and fun, you have to think of it differently in high level campaigns. Going back to the dungeon metaphor, it's no longer enough to simply populate each room with a bad guy, and have their existence be all the justification the PC's need to adventure. Instead, there simply being a bad guy is no longer sufficient reason for your PC's to adventure (they're probably just teleport in and kill him, game over). Instead, you need to focus on the 'why' of the adventure. Why are your PC's getting involved? What is their motivation?
That is the crux of a high level campaign. You have to stop thinking in terms of dungeons, and instead think in terms of an open sandbox, where the PC's personalities, history and personal motivations drive the game. When the decisions are moral ones, when the outcome of the game depends on the integrity of a PC's character, their ability to teleport will not change a thing, and suddenly the intensity is right back on.
Source by Phil Tucker