As the article mentions, Thunderbird as an email client is _okay_. Rewind back to the late 90s, and applications like Winamp were pretty amazing, it just needed a better user interface. There are loads of music applications on Linux, but I end up using CLI mplayer and even Foobar2k under Wine, because of some of the other shortcomings.
Even the core of Linux desktop applications like Nautilus feel like they are regressing. When a file manager in my mind should be at the heart of an OS. Mounts and copying should be very intuitive.
Much functionality exists in CLI apps, but is stripped out or lacking in desktop applications. Having something like Rsync in your file manager could be great. And better integration with something like Git. But that's after the core utility of the application is polished to the max.
It's the final 10%, and I appreciate how difficult that is, with moving targets weird Window tool-kits and other abstractions and APIs, but it's that polish that makes it.
Having said that Windows always feels like a fragmented Frankenstein's monster with UI inconsistencies not only across core applications but also with third party ones.
While some of the desktop paradigms have worn well, I don't see much radical experimentation on the desktop, and Linux could provide exactly the platform to try things out.