Here is a nice article by Colin Moock, discussing some issues that people have been facing while learning AS3. Below is the list of charges:

1. The removal of on()/onClipEvent() from Flash CS3 makes creating simple interactivity hard.
2. Getting rid of loaded .swf files is hard.
3. Casting DisplayObject.parent makes controlling parent movie clips hard.
4. The removal of getURL() makes linking hard.
5. The removal of loadMovie() makes loading .swf files and images hard.
6. ActionScript 3.0’s additional errors make coding cumbersome.
7. Referring to library symbols dynamically is unintuitive.
8. Adding custom functionality to manually created text fields, to all movie clips, or to all buttons is cumbersome.
9. The removal of duplicateMovieClip() makes cloning a MovieClip instance (really) hard.

Read more here.