Setting aside that most windows client development should probably be done in WPF/C# these days... low level differences: There are differences between GDI an

Setting aside that most windows client development should probably be done in WPF/C# these days... low level differences: There are differences between GDI and xlib. For instance GDI has a concept of a "combo box" at the API level and in X11 you need to either roll your own or use a higher level GUI toolkit. IIRC, Handling of the non-client area and window scrolling are different between platforms and handling of audio/video is completely different between platforms. There are a LOT of features: Power modes (e.g. suspend, hibernate, low battery), tooltips, the clipboard, drag and drop, maximize, and a bunch of subtly different "must have" features need to work between platforms and you need to know what all those features are. Accessibility should work and touch/gesture support is likely to become important over the next few years. At the point where you start cutting features to make it more manageable you have started down the road of making another crappy "lowest common denominator" GUI toolkit and you are better off just using QT. User expectations are different between platforms: This is mostly a lot of little details, like where the "close" button goes, what a "right click" does, menu layouts, standard icons, and which direction the window scrolls when you move the mouse wheel "up" but it adds up to an application not feeling right. You can see this taken to an extreme with any number of bad ports from console to PC.