One of the biggest difference I found migrating from AS2 to AS3 is strict data typing and better error messages. I have had less silent failures. Not that I *like* errors, but I *like* errors.
I found this link useful when I get confused by some compiler errors in Flex.
http://livedocs.adobe.com/flex/2/langref/compilerErrors.html
These are my favorite ones:
1046 Type was not found or was not a compile-time constant: _.
1118 Implicit coercion of a value with static type _ to a possibly unrelated type _.
This link is also useful on runtime errors in Flex.
http://livedocs.adobe.com/flex/2/langref/runtimeErrors.html
This one is my (least) favorite:
1009 Cannot access a property or method of a null object reference.
This link is on Flash 9 compiler errors:
http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/compilerErrors.html
The migration has made me feel programming is more about debugging than writing code. :S
