A tech-savvy artist, who would previously have needed a handful of devs and a publishing company to get a fun toy to a few hundred users, could suddenly build and send something around the world in a week. No wonder it uncovered so many creative projects waiting to happen!!
As someone who wrote a complete 3d modeller for a game, including animation, even before 3dsmax was a thing, I was surprised when a few years later (10+) I saw how easy the Flash UI was to understand, especially the way keyfames were added. The UX was rather extraordinary. Unfortunately even at that time (early iPhone) because Flash was so synonymous with security flaws, I would regularly uninstall it. Since then, I've yet to see a solution that's as easy to use and doesn't have some kind of vendor lock in (like Flash did). Maybe I haven't looked hard enough tho.
As much as I personally disliked the artistic Flash- only sites at the time, hindsight shows that they were rather invaluable. We lost a lot of diversity when Apple killed it off.
It's easy to imagine the various security vulnerabilities could have been fixed by a more responsible/responsive company, and the abuse by advertisement companies should've been solvable with an ad blocker.
JavaScript, CSS, and HTML have come a long way in capability, but the mashup is much more distasteful to me. Flash was elegant despite its frequent misuse.