Allowing for multiple tasks to be done at the same time. Whether it be skills or keeping track of other players/npcs while playing, the player should be able to complete the game without losing track of what is going on.
The game roller coaster tycoon makes the player keep track of an entire amusement park. They have to constantly check to see if rides are breaking down, trash is being thrown around, lines are too long, debt, promotions, lines, everything that happens in an actual amusement park. The player has to be on top of things and know whats going on in the entire park so it can run smoothly.
Patterns - Enemies, surprise, strategy, changing game environment
Lenses - interactive sound, challenge
In Halo Wars, players must command an army to defeat the enemies army. You can get multiple bases and play with up to 2 other team members. Players must be aware of all of their bases as well as their units, if they split them up. If they are playing with other AIs they should know where/when they are attacking or being attacked as well as their leader powers. All of this information can be a lot for a player playing with AIs, but once they get to multiplayer it becomes very intense. This holds true for almost all real time strategy games.
In League of Legends, players have to pay attention to many parts of the game at the same time. They have to worry about their health as well as magic at the bottom of their screen. They also have to pay attention to the map and know how each lane is doing. Then they have to pay attention to their screen and know what the NPC enemies are doing as well as other players. This is done by paying attention to the map as well. So you are constantly moving around and trying to keep track of everything to do well.
How many tasks are too many?
When designing your game, make sure that you aren't overwhelming your players. There can be a point when there is just too much going on in the game for them to take in all of the information in a timely manner. Any unneeded information or actions should be gotten rid of to keep the gameplay as simple as possible and easy to understand and divulge.
Keep them simple!
Some games are known for having a single task at a time that is really complicated. Like Street Fighter or Tekken. They have only a single task at a time but the user must know complex combos to succeed. But if you are designing a game that is utilizing many tasks or information at a time or all the time, you must make sure that they are simple. Like health or magic levels, they convey exactly how much you have left so you know what you can use. This allows the developer to add in more tasks without fear of overwhelming the user.
Multitasking in games is a standard in the industry. It's just a matter of tweaking it to your game and how you want the user to multitask.
Motivations - Competence
Components - Information