I think that most of the other information is available through one method or the other already. Supported languages can be obtained through Portletconfig.getSupportedLocales() and the portlet name through PortletConfig.getPortletName(). The supported processing and publishing events are available along with the supported public render parameters. The portlet title, short title, keywords, display name, etc. are available through the resource bundle (even if no real resource bundle is provided ).
What I find to be missing completely is a way of obtaining the defined and supported portlet modes and window states. You can get that information sort of indirectly through use of PortletRequest.isPortletModeAllowed(PortletMode) and PortletRequest.isWindowStateAllowed(WindowState), but you have to sort of try it out ... it isn't very elegant.
So I think we need a way to get an enumeration of the available portlet modes and window states.
It might be that we need other information in a nice API as well, but so far I haven't found anything else to be missing.