Evolution of the Windows Interface
The evolution of the Windows interface can be summed up by the type of standard controls provided by the OS. Most of the time the incorporation of these controls into Windows was preceded by their introduction in Microsoft Office applications.
Windows 2.x has pretty much the standard dialog controls like labels, textboxes, listboxes and buttons. Windows 3.x then brought a 3D button, comboboxes.
Windows 95 introduced a slew of new 3D controls, such as rich text edit, toolbars/statusbar, progressbar, treeview/listview, shellview, animation control, property pages and wizard controls. Most of these interfaces were adopted from Office 4.0 interface.
Windows 98 included an embeddable IE control that made it easier for applications to render html views. This control became so pervasive that I believe nearly every application hosts IE; in Excel, for example, uses IE to help locate and extract refreshable data from standard web pages. ActiveX controls, which many new system technologies such as Microsoft Agent were exposed as, also came to life.
Windows 2000 and XP really did not add too much, though we did get special effects like GDI+ gradients and alpha-blended layered windows.
In Avalon, I expect to see more controls, though we might actually lose a few like toolbars and statusbars in the new presentation model. This time, we see a reversal of roles, with Windows, not Office, leading the advancement of the interface.
All the controls, like video, will have the same managed programming model and be better integrated with each other, whereas before some controls were ActiveX controls and others were standard window classes. So, it will be much easier to integrate traditional controls, media, and document.
Each control can apply linear transformations, perform rich animation, and display without flicker. Traditional controls will have more extensibility. For example, the contents of a button can be any set of controls, not just text. One of the tenets in the Avalon group, that OS developers should dogfood their own extensibility model; this prevents the forking into two interfaces that occurred in the past where one extensibility model existed internally for OS developers and a crippled, often useless, extensibility model existed for outside developers.
Longhorn will add support for rich vector graphics. There are new panels to automatically layout controls. There is much better support for flowing text; the text layout capabilities, which include wrapping and multiple columns, is targeted to be nearly equivalent to that of Word. This reduces the cost of building full wordprocessing features into a product.
The navigation-based UI means we should see address bar (also known as breadcrumb bars) controls. The new collaborative orientation will bring a formal concept of a person or "contact" to OS; there is specifically a new contact control. There are other concepts, which Microsoft feels are imported, being formalized, complete with a standard WinFS item class (like Location, Annotation and GameLibrary), and there may be a control for those concepts as well. Looking around the Longhorn MSDN site, I see controls like CalendarControl, Clock and BasketControl.
There are a few more controls geared to a more appealing visual presentation, the most famous being a rotating carousel view. I guess ListViews are passe, now.
There is a link with graphical screenshots of various versions of Windows over time.