What do you think would be the key factors to drive mass adoption of D?

[Lance Bachmeier]

This has been discussed many times before, but I'll offer a few 
thoughts.

It's a fantasy to view D as having any chance of widespread 
corporate adoption. That would require tooling, libraries, and a 
monetary advantage to adopting it. If you're choosing between 
Java, Go, and D in a corporate environment, it's hard to see that 
you're choosing D.

The places where it could be adopted is scripting, medium-sized 
solo projects, data analysis, and that sort of thing. Single 
developers using the language they like best. Unfortunately, 
that's never been a priority of D. There is no interest in 
evolving the language in ways that appeals to those users.

> -   A truly ****batteries-included IDE**** (something on the 
> level of Visual Studio for C# or IntelliJ for Java) — would 
> that make a big difference? Right now the experience is mostly 
> VS Code + code-d + serve-d, which is decent but still feels 
> fragmented compared to mainstream languages.

This would definitely help.

> -   For corporate/enterprise environments in particular: do you 
> think a modern ****GUI designer**** (visual drag-and-drop) 
> would help a lot? Many teams still build desktop/business apps 
> and love tools like WinForms Designer, WPF/XAML preview, or Qt 
> Designer.

The first question to ask is why anyone would choose D over the 
alternatives. It's the "nobody ever got fired for buying IBM" 
thing. Corporations already have languages that work.

> -   Strong ****corporate backing**** (big companies using + 
> sponsoring it)

We have some of that already.

> -   Way more high-quality ****tutorials****, beginner-friendly 
> learning paths, and real-world project examples

Yes. This would lead to adoption in the areas I mentioned above.

> -   Active ****evangelists****, conference talks, YouTube 
> content, and community momentum

Ditto.

> What else do you see as important missing pieces? Better mobile 
> / web / embedded? GC improvements or u/nogc-by-default push? 
> Something completely different?

Web could be a growth opportunity but it's extremely competitive. 
Anything nogc is a dead end. Rust won that game. The game's over, 
the referees went home, and the janitors have locked the doors.