You could develop using all the powerful Visual Studio tooling. It didn't crash all the time like Flash.
Using Silverlight and XAML was sort of like what I imagine web programming will be like in about 10 years. Except that you could do it back in 2007.
You have a high level language (C#) with reusable components (UserControls) and fantastic debugging, profiling, static analysis, and automatic refactoring.
Incidentally, that is where the web is just now heading. Higher level and strongly-typed languages (AtScript, Typescript, Flow, ES6+) with reusable components (Web Components) and better tooling (Firefox and Chrome dev tools)
They never have in the history of their company so I'm not sure why people believe this time they will. Open sourcing the core guarantees nothing if they move the Microsoft support at a faster pace and don't commit to keeping the other platforms at par.