In “On Browser UIs”, Richard S. Tallent has a comprehensive list of reasons “why web apps suck". On “missing widgets":

“Tree views: even the best ones out there suck. MSDN, for example. They also almost invariably lead to using frames to avoid population and rendering on each page refresh. […]

Context menus. This alone is responsible for any number of cases of over-busy interfaces in web apps. Without the right-click ability (sorry Mac users), the interface must display every possible action and provide an alternate method of selection. Combined with a lack of drop-down menus, the result is a busy interface that must show too many possible-but-unlikely actions.”