It's funny: the security aspects never really bothered me back in the day (although they should have done).
No, the reason I looked down on Flash, was because all I ever saw were CPU sucking ads, fairly boring non-interactive presentations/videos, those dreadful websites that were all Flash and took an age to load, and then the crappiest of crapware games with no love or attention to detail put into them. As a result, I just came away with the idea that you couldn't do anything good in Flash.
I was wrong. Incredibly and spectacularly wrong. Ironically so, because I'd been a big fan of AMOS back in the day on the Amiga (though I'd have been better off with Blitz, probably) - another environment designed to make it super-easy to take advantage of the multimedia, animation and gaming capabilities of the host platform.
Don't get me wrong: you can do great stuff with HTML5, JavaScript, CSS, WebGL, and so on, but with Flash you could do all that stuff 15 years ago, and it made it all just somucheasier. So with the passing of Flash I can't help but think we've lost something, and it makes me a little sad to think about it.
Yeah, I have to agree with you there that the web platform has not _quite_ caught up with the total ease of Flash. I think it's starting to get there within certain domains, though, but it has been a long time in coming.
The Flash authoring tool was always, imho, pretty nice. The runtime had lots of issues, though.