Full C# instead of god forbidden js.
Full vector dpi aware UI, with grid, complex animation, and all other stuff that html5/css didn’t have in 2018 but silverlight had even in 2010 (probable even earlier).
MVVM pattern, two-way bindings. Expression Blend (basically figma) that allowed designers create UI that was XAML, had sample data, and could be used be devs as is with maybe some cleanup.
Excellent tooling, static analysis, debugging, what have you.
Rendered and worked completely the same in any browser (safari, ie, chrome, opera, firefox) on mac and windows
If that thing still worked, boy would we be in a better place regarding web apps.
Unfortunately, iPhone killed adobe flash and Silverlight as an aftermath. Too slow processor, too much energy consumption.