I'm glad you noticed the difference
For WM5, we made a huge investment in re-engineering how we handle with compatibility during the development cycle.
Aside from finding and fixing a ridiculous number of backwards compatibility related issues that users will never know ever existed, we determined the exact causes for applications not running correctly on WM5 in cases where we couldn't make OS changes to fix them. Those details were published in the Migration FAQ on MSDN back in May and many ISVs were made aware of issues specifc to their applications long before WM5 was even announced.