User friendly, consistent and responsive is what we want. Looking good is also important.
There are benefits having the user interface described in a user interface language. We are in paticular looking at the XUL language, but instead of javascript, we want to use Ruby.
Such language makes it easy to do internationalization (i18n) of the user interface. For instance a sentence of a few words in one human-language, can span over many lines in another human-language. Therefore sligth modification to the user interface are often needed.
Looknfeel are very flexible. Perhaps there are too much sun light in your room during the day, so that you want a high-contrast theme. In the night you want to be easy to your eyes, and thus uses a low-contrast dark theme. Maybe you have a 28 inch display at your home, but only a 14 inch at your work. Its easy to change the size of the font, so that applications appear to be same size.
Rapid Application Development are possible. Because the user interface are defined in XML, it can be created via GUI-builders. You can let your webdesigner create nice visual appealing interfaces, no need for expensive programmers.