[ori_b]

It's also widget selection; On Android, for example, you may want a hamburger menu. On KDE, your menus get reshuffled compared to your OSX or Windows UI.

I've seen plenty of systems that allow you to do mediocre UIs on all platforms. I haven't seen one that allows you to do an excellent UI without doing once per platform.

[kitsunesoba]

I think the answer here is to have a "base menu" that all applications start with instead of starting from zero and having developers fill in the blanks. The base menu knows where to put common special items (Preferences/Options and About for example) on each system, and developers build on top of this by adding their own custom items.

I'm not saying that it's possible for a cross-platform UI toolkit to be perfect, but it's certainly possible to get a lot closer to right than current toolkits do.