boltzmann's, yep. The killer feature of Flash was not the rightfully universally hated player. It was the authoring software. You could make really cool things really easily with no programming whatsoever. For even cooler things, you could copy-paste snippets of ActionScript from a forum or something, without having to understand much about them (gotoAndStop(frame) is kinda obvious to anyone). And in the end you get a neat single .swf file that contains your entire animation with all its assets. Just drop it to your server over FTP or something and you're good.

Compare this to modern "replacements" that involve writing non-trivial code for a hello world and using command-line tools to build it.