What you are looking for is preemptive multitasking
Multitasking (Windows)
which the Windows Phone kernel is capable of, but for some reason limited on Windows Phone. My guess is that its related to resource management. I believe that once you start seeing 4000mAh batteries and 2-4GB RAM as standard they will unlock the full potential of the system...just like what you see on Windows for desktop.
There are only a few mobile operating system that act like desktop ones when it comes to multitasking..
QNX (BB10), EKA2 (Symbian), and a Linux distro called Maemo/Meego which is now Tizen.
Android, iOS, and WP are all inferior from that point of view.