- Battery life - many applications running in the background will drain your battery really fast.
 - Available memory
 - Processor 
 - Small screen
 - Did I mention Battery life? :)

Here is an article about this feature on the Windows Phone Platform. It starts by explaining the reason why this it is a "necessary evil": 
[Mobile Matters - Windows Phone 7 Tombstoning](http://msdn.microsoft.com/en-us/magazine/hh148153.aspx)
> A good mobile platform should acknowledge the hardware constraints that mobility imposes on the device. Compared to desktops, mobile devices have less memory, less processing power, limited screen real-estate and limited battery life. Add up these constraints and you should conclude that, on a non-dedicated device where many applications will run, applications will eventually be closed or shut down to make resources available to other applications...