For example, it does excellent semantic syntax highlighting where the same variable is drawn in the same color throughout - after using it for a while, code in other editors looks pretty dead :-).
And FWIW, considering that KDE apps do work on and get shipped for Windows and Mac OS X, that "only platform" is a pretty broad one. Actually I can't speak to how complete the port of KDevelop's own application logic to those platforms is as I haven't tried it there, but I certainly happily use apps like Okular and Gwenview on Windows 7. Here are shots of that too, while we're at it: http://simplest-image-hosting.net/png-0-windows and http://simplest-image-hosting.net/png-0-capture24
That shows some of the issues I have with it - I don't like the custom theming, especially given how inconsistently it's applied (still uses system looks for scrollbars, the menu bar doesn't fit at all, etc.).
I really wish the various teams working on open source IDEs would work together to make one very good IDE with a good plugin infrastructure to support the union of their features, instead of a zillion quite-good-but-not-it ones.