Great all around, code is readable despite some calling that verbosity. A lot of discipline is required to not end up with spaghetti in entreprise environments. Ecosystem like none other. Java 8 was a true hope when it comes to belief in its future. It is here to stay and that is a good thing.
One day I will write down the million things I would have to say on it.
Groovy: Underwhelming is the word. It doesn't offer nearly enough to warrant moving to it. It is a cool language but being cool is not enough the competitive environment programming languages evolve in.
Scala: Was the cool one until Java 8 got out and basically stole a lot of the appeal that was in the functional style of Scala. Naming decisions have also been misguided if the expectation was for it to become an industry success. I hope Dotty saves it and that it learns a few things from Kotlin.
Kotlin: The people are Jetbrains are obviously amazing at both programming and marketing. And they really outdid themselves with this one. The syntax is very appealing and straightforward and the tooling is only second to Java and C#. Android support by Google has and will keep skyrocketing its adoption rate but also might make it sound like a mobile-first language when really its place is also on the server.
Python: Dynamically typed so tooling is forever doomed. Object model is just not up to par with other languages. The library system could be great if pip was better. Insanely good as a build system DSL. Again, why is Jenkins using Groovy?
C#: LINQ is quite the wonder to use. UpperCamelCase is simply not ok. Closed source is not ok either. Maybe .net Core can salvage most of the downsides. That would actually be a great thing.
JavaFX: It is unfinished and as of now not good enough. Maybe the community can fix it. I hope so at least because it could be the one real alternative to JS+Electron. It is although doubtful that there will ever be enough community support.
Electron: it is Windows Vista. Very pretty for the time but I am happy it is not yet ubiquitous because RAM prices are still stratospheric. Community is so vibrant it will probably find a way to fix most issues.
GTK: You would not write a GPU driver in Java, and you should not use C for GUI.
C#: I heard it is great. And for the little I used it it was pretty good. If you only target Windows it is probably BiS.
Disclaimer I have an immense respect for what people can build on that side of the software industry, but I cannot get myself to enjoy it.
AngularJS: A good idea. But it is dying so there's that.
Angular/React: Never really got into these. Not a big fan of the abstractions they're based on.