As Kulnet said, it's the way memory is managed that nakes WM5 so attractive. In WM2003SE and below, RAM is used to run AND store programs/data. What this means is if your device ever loses power (e.g., your battery drains completely, your device hard resets), everything you have installed/saved in RAM (also called Main Memory) is lost. With WM 5, this wouldn't happen since programs/data would be put in the ROM (also called Built-in-Storage), which is untouched by power loss. This also helps to conserve battery life since the device doesn't need to use power to maintain all the stuff in the RAM.
Take a look at this thread for more comments and a link to a good review to WM5:
http://www.aximsite.com/boards/showthread.php?t=97182
Moved to WM5 forum.