XML Syntax
Makes documents easy to write, edit, and parse.

Flexible Box Model
Recognizable to those familiar with GTK, XULÕs flexible box model allows controls to expand to fit available space and resize automatically with windows. Comprehensive flex and orientation controls allow for the development of applications that handle large amounts of data and UI that Òjust looks rightÓ without having to write lots of code.

Wide Array of Application Controls
Standard set of controls expected of a platform including buttons, menus, toolbars, tree and list controls, a canvas control for rendering, etc.

Data Binding
Currently through RDF, in the future through a new storage API which will support SQL based selection queries, etc.

Rich set of platform services
Facilities to handle networking, file I/O, preference read/save, and numerous other common tasks.

Cross Platform Development
Develop on any of the Tier-1 platforms, Windows, MacOS X and Linux, code can also be targeted at a variety of ports including BSD/Unix, OS/2, BeOS, Solaris, HP/UX etc.

Scripting/Language of Choice
XPCOM components can be implemented in C++, Objective C++, JavaScript, Python, etc. Scripting in XUL is done using JavaScript, a flexible scripting language that makes numerous common tasks simple.

Sec508 Accessibility
XUL Widgets are Section 508 accessible, and tools are available to allow new widgets you construct to work with screen readers and other accessibility aids.

Localizability
Strings stored externally in entity files/properties files with utilities for formatting and reading

Tested and Deployed by Millions
Firefox 1.0 downloaded over 50 million times so far. Firefox 1.5 will include the XULRunner platform.