Musts:
Object-oriented programming and design
ActionScript 3
Flex 3
PureMVC (or other MVC frameworks)
Flex component skinning
Pluses:
CMS (Content Management Systems) experience
Some graphical skills (Photoshop, Illustrator)
Flash platform (understanding of how the player/VM works at a high level)
Understanding of Agile methodology (SCRUM, Iterative, etc)
Other minor pluses:
Experience or interest in Mobile development
Technical Architecture
Design Patterns (Command, Decorator, Strategy, etc)
Previous Flash experience