Introduction
Gameflow is a significant element because it directly relates to how your map plays and how much enjoyment players will get out of it.

The six main issues to bear in mind when designing a level that has good gameflow are...

Height
Different height levels make for much more interesting and varied fights. Battling on one level plane only offers so much.

Balance
Essentially, anything in the map that gives the player a advantage, there should be a disadvantage that evens the playing field.

Variation
Corridors and rooms that are all the same, whether it be in looks or size, are boring and confusing for the player. Having a varied structure to your map means the action in your map will be varied.

Focal Points
There needs to be integral points within your map where players ultimately converge. At the very least they provide somewhere for the players to orientate themselves, which also ties into having variation.

Flow
Your map should always encourage players to be on the move (ignoring specialist situations like snipers, campers etc). They should rarely have to backtrack to get around the map.

Choice
Players like choice. They don't want to be limited to the same routes as its boring and limits tactical options.

Example
With those building blocks introduced I will now expand on them by applying them to a basic map layout...

Though this might look incredibly boring, it does demonstrate 2 of the building blocks to gameflow (albeit on a very basic level). No one side of this corridor offers players a extra advantage so everyone would be on a balanced playing field. Also players would be constantly going round and round which creates a sense of flow.

Not much has been added here but what we have started to create is a bit of variation. The corridors are now different lengths and the map is no longer symetrical. This is significant because we have created different length corridors and that caters to a larger variety of players and weapons.

Again, a basic change but a significant one. The players now have choice. Do they turn left, right or straight on? Arguably you could say they had
choice before in whether they went clockwise or anti-clockwise around the map but that is such a insignificant decision for the player it would hardly matter.

I've now added a large room to the map which adds the focal point. All corridors now lead to this point and players should now easily be able to orientate themselves. The corridors follow different shapes and join to the room on different sides.

Finally I am going to add a element of height to the map by adding a platform in the main room. This is significant in a few ways. Besides adding height it also creates some variation in what the fighting will be like in that room. In some ways it also adds a focal point because players will fight over getting the height advantage.

Notice that to keep a sense of balance I added two seperate stairways to the platform. Though a player may get the advantage of being higher up than other players he will have the difficult task of defending this advantage from two directions. It also helps with flow because otherwise there would be only one way up to the platform and the player would have to backtrack to leave it. Finally I added a small room to what was otherwise a long boring corridor, again for variation.

Supplies
A comment should also be made on power ups, health, weapons etc. These have a big influence on balance and focal points because in a sense they themselves are focal points for the player. Taking the example again, if supplies were added to the main room then there would be little reason for the players to leave it making the corridors largely redundant and effecting the flow of the map.

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This article, and others, can also be found on my website, Creating Worlds