There are classes and functions that should be avoided in KDE applications, but KDE has no direct control over them (for example, they are part of Qt).
KDE applications should not use Qt classes that have their KDE replacements. Examples include QFileDialog, QDialog, QMainWindow. KDE versions of these classes provide better integration with KDE.
For frameworks, a lot of functionality from K classes has been merged into upstream Q classes. Use of QFileDialog, QDialog, and QMainWindow are ok and even encouraged, while other classes like KLineEdit and KComboBox are only required for utilizing features like completion See Frameworks Porting Notes for more details.
Use KNotification for better KDE integration.
QHttp does not respect desktop settings such as the user's proxy settings or resource restrictions. Always use KIO to access network resources.
Try to avoid calls to QWidget::showFullScreen(), QWidget::showMaximized(), QWidget::showMinimized() and QWidget::showNormal(), use them only if you understand what they do and you really want that. These calls do not only change the relevant window state as most people expect, but they also have other effects. For example, showFullScreen() among other things also removes the maximized state (see bug #157941), the same way showNormal() is not an inverse call to showFullScreen().
Either use QWidget::setWindowState() in the following way
widget->setWindowState(widget->windowState() | Qt::WindowFullScreen); // set widget->setWindowState(widget->windowState() & ~Qt::WindowFullScreen); // reset
or use KDE API - KToggleFullScreenAction has setFullScreen() helper function, and KWindowSystem has calls for changing window state.
For further programming tips, please read the article about common programming mistakes.