1/ Its IDE which made creating creative and interactive content mixing video/vector graphics sounds and code easier
2/ it's API that provided things that were not possible when browsers had none of these API (video streaming, socket programming...). At some point you could even write 2D shaders or inject C/C++ code with something called Alchemy.
Point 2 is now largely covered by browser web API, although one might argue that the performances might not be always as good as they were in flash, especially when it comes to vector graphics or realtime audio processing. But 3D and webgl are more performant than flash in the browser.
Point 1: well there isn't really an equivalent, and even Adobe Animate isn't really doing exactly what flash did when it comes to authoring content for browsers. So there is still a potential market here. I'd like to see something node based when it comes to coding. Artists love their nodes.
The real problem is obviously running old flash content like games. Some of these games were really good. I remember playing one which was a hotel management simulator and it was really really fun. But it's a bit like a these jar games in the 2000' one cannot run on modern mobile phones anymore. Without an effort to preserve these, they will all be lost.