Generally the UI is written with one of the two big GUI toolkits: GTK (Gnome) or Qt (KDE). If you run Gnome, you get GTK and all the apps work without needing to download anything else, but if you have Gnome and want an app using the Qt framework, you'll need to download the Qt libraries, which can be substantial. Even after downloading the Qt libraries, though, the application might still look weird, since it's not a native GTK UI.
The state of WMs on Linux is really, really more complicated than it needs to be.
> The state of WMs on Linux is really, really more complicated than it needs to be.
Choice is always going to be a prominent force in Linux. There will always be folks who are going to write new toolkits and apps and there they will find users who would want to promote it. That is just the nature of Open source.
That being said I think no one really likes to use GTK+ app if they are targeting multiple desktop environment and operating system. Ubuntu's Unity 8 and KDE both are using Qt and Qt is already available on all possible platforms including Android.
I haven't done a whole lot of GUI development, but I briefly looked at Qt and it seemed like it cost a huge amount of money to use, so I wrote it off. Is that not the case?
The state of WMs on Linux is really, really more complicated than it needs to be.