In a session on background tasks in Windows phone 8.1 it was revealed that users will now have much more control over when and how many background tasks run.
The maximum number of tasks that can run in the background is now 20, but by explicitly enabling an app to run in the background this number can be increased to any arbitrary number.
In addition users will be able to set an app as being able to run in the background, even under Battery Saver mode.
The short of this is that under Windows Phone 8.1 the users will have much more control.
The full session from BUILD 2014 can be seen here.