The Yorba Foundation is doing some pretty interesting work with limited resources. They write well-designed apps with attention to the UI.
Shotwell and Geary are both written in Vala, a C#-like language layered on top of the GObject system. It will be interesting to see how Vala matures. It has always seemed like a messy abstraction, building it upon GObject, rather than its own type system.
Why should it build upon yet another type system? Vala was designed around GType for a reason, so that it would not have to deal with the same language-interop issues that occur with languages that use different type systems from GObject. Vala originally compiled into C, so implementing another type system in C that isn't GObject would be kind of silly.
If you develop in C#, Java, Python, or Ruby then you need a layer between GTK and your language which can make their type systems work together. But Vala was designed around the existing GObject system, and since it compiled directly to C it didn't need much in the way of a glue layer, at least for GObject-based libraries.
In fact, Jurg, the creator of Vala agrees on that enough that he created Dova: https://live.gnome.org/Dova it's no longer actively maintained, but it stands to illustrate the point.
Shotwell and Geary are both written in Vala, a C#-like language layered on top of the GObject system. It will be interesting to see how Vala matures. It has always seemed like a messy abstraction, building it upon GObject, rather than its own type system.