For me it is the language it's written in: Rust. Now I can participate, fix bugs and implement new apps with the language I know the best.
Some people might also say less crashes, less vulnerabilities and all that, but for me the first part is the most important.
Yep, but QT's object model and its being written in C++ makes it super cumbersome to use in Rust. GTK is better here due to it being written in C, but the direction it's taking in GTK4 is not really great, and having a safe Rust UI toolkit is a huge win for the community.
Cosmic being fully Rust means I can just take one project from them, and immediately start working on it with cargo and all the familiar tools. It's not as easy with C or C++ projects in Gnome and KDE.
I think it's great we have some competition in this space, everybody wins.