Right now it is possible to add tooltips to action buttons in the modeler.
It would be nice if this was extended to other Mendix widgets so that tooltips can be added to reference selectors, date pickers, check boxes, radio buttons, etc.
As HTML5 explicitly allows the "title"-attribute (which generates the tooltip in the browser) on all HTML elements: yes please add this to the standard widgets.
Sometimes you need to use abbreviations when rendering text, labels, etc and it would be very nice to be able to add the full name as a tooltip on mouseover.
It is user-friendly, makes the ui more self-explanatory.