I wonder though, what made Delphi so simple in terms of UI development? Is there anything that could not in theory also be applied to other languages, e.g. Java?
One of the features of the gui components that we have used extensively was the double data bind. It is extremely powerful to be able to connect various input fields to a data model and have the results updated automatically upon changes.
A very good IDE with first-party GUI designer integration, and a big ecosystem of third party components for just about anything.
The language itself is not impressive compared to today's options.