1. Creating Hello World was dead simple. Just like photoshop, it was as easy as selecting the text tool, typing out what you wanted and hitting publish. Instant visual feedback for what you were creating.
2. Flash swf startup is measured in milliseconds, java applet startup is measured in seconds. Sun never seemed to care about "visual java" and they suffered for it.
3. Single vendor control. For years Macromedia and Adobe were giving devs and designers the apis they wanted, rather then being stuck in design-by-commitee hell.
5. Absolute layout. While html developers were complaining about box models, flash developers were building things to look exactly as they wanted them to.
4.1 Who blocks JS without also blocking Flash?
5 This is what I'm talking about. Flash devs were busy building exactly what they wanted while HTML developers - by virtue of not using absolute layout, etc, were delivering what users could actually use.