Flash was great, but abused to do bad things (like all good tech)

The killer was the iphone not being powerful enough/having enough ram to run the plugin, and adobe refusing to make concessions.

What it got right:

Design once, looks the same anywhere

reasonably powerful scripting language

Vectors as a first party drawing primitive

abstracted OS hooks

This was it's downfall, because it was for the time heavy to run. Combined with advertisers wanting rich flashy adverts, meant it became the bane of people's life.

There is still no replacement that is easy to author, and works pretty much anywhere. Sure there are loads of JS frameworks that sorta do one part of what flash did, but none of them have the rich editor that allowed you to have such creative freedom.

The closest thing to it now is unity.