file open/save dialogs, i18n, drag and drop, image formats, web views, any sort of accessibility, themability that actually fits into native platforms, sometimes the ability to open new windows, many common controls that we just take for granted now that you have to implement yourself, sometimes limited amount of events you can reasonably handle based on the design of the api (IM gui's for instance).

I wouldn't say never use an minimalist framework. It can be handy for embedded debugging tools or for toy projects. But I wouldn't use one of these things for an application that I thought might have a few thousand users and some sort of life span.