From Mods Repository Wiki
How fast runs the map by an excelent portals work, how are the obstacles placed around the hole map, how good are the mantle textures used to effectively avoid them, how well are the models Clipped so player don´t get stock and how are the spawnpoints set in relation to the objectives. all those points and many more, defines whether or not you map have a good GAMEFLOW.
Another very important point, that you should really care when building a map and you can notice is present in most of the stock maps is that you MOST create the necessaries ways for the player complete surround the entire map unexposed or Barely being seen by others, or just cross it trough the middle from one side to the other if wants.
Here are some examples from COD4:
Drop some lines on paper before start mapping, helps alot to avoid those annoying BOTTLE NECK. You most take some time and design how and where you want the players to move around your map, in other words, design the GAMEFLOW.