Just the sheer amount of interesting games and videos available on places like NewGrounds and Kongregate was something else. It was kind of crazy how easy it was at the time to get at least some attention to any proof of concept project simply by posting onto said sites.
Something that has definitely gotten much harder over time despite the more advanced tools that are available to hobbyists these days.
I think it was a mix of how easy flash made it to go from some vector art drawn in app, to hooking up some key-frames together with logic for a game etc... But it was also IMO strongly influenced by the internet culture at time, where there was (at least it felt like) a larger audience of people available to try out wacky often no/low-budget things.
I haven't quite seen the same environment emerge on places Itch.IO and to an extent Steam.