Just the Eclipse legacy and development approach perhaps. Eclipse is 24 years old and its codebase stems from VisualAge before that, so I imagine it's ... idiosyncratic. Probably the most successful thing built on OSGi, though I dont imagine Theia brought any of that along.

At least for a while, Eclipse seemed like an Architect Astronaut's happy fever dream: actual bits if implementations hidden behind 5 to 10 nested interfaces and facades wherever I looked. I remember that at one point that I wanted to debug some weird behavior in the Plugin Development Kit and after sinking about half a day into exploring the source code, I didn't even see a single line of code that was actually doing anything meaningful. It was quite shocking to my old junior level self.

Eclipse Theia is based on VSCode, not the legacy Eclipse Java IDE.

Well, it uses the just the Monaco editor, but the rest is not based on VS Code.

I am not fan of Eclipse, mostly due to bad experience with it, but this is excellent idea, if more people and companies would invest in it's development, we would have alternative to VS Code.

I had the displeasure of using Eclipse in the 2000s Java times. I don't have many fond experiences of it.

We finally discovered IDEA and never looked back.