Qt has been indispensable to my daily work. When I first started my current job, I had to convince my boss that we should transition from C# to PyQt (and also the rest of the Python ecosystem, which is awesome because we do commercial scientific computing). Mixing Python and Qt has been great for productivity, as you can imagine. There is also a great Python visualization ecosystem based on PyQt/PySide including pyqtgraph and vispy. Lastly, the documentation is absolutely outstanding, and the community is very active.
A project I have been wanting to start is to use Qt as a substrate for a (functional) reactive programming framework in Haskell or Python. The Haskell lib could be based on any current FRP implementation, and the Python lib could be based on RxPY [1].
One thing I would change in Qt would be the Model/View system. I try to use a strict Model-View-Presenter approach, but since we have millions of rows in our tables we have to use Qt's Models/Views instead of controlling all view behavior ourselves. It would be nice to have performant views easily while using a different API, instead of being forced to use the QAbstractItemModel API.
Congrats Qt project, and thanks for the hard work!