One of the changes in the Gnome 2.9 development series is the removal of most of the Bonobo code from Nautilus, which results in a speed boost due to lower complexity and less IPC overhead. This had the effect of breaking existing bonobo based context menus, property pages and views. The first two can be converted to the Nautilus extension interface, but the second has no equivalent in the new code (partly because Nautilus is concentrating on being a file manager these days rather than a universal component shell like it was in the early days).
Two of the casualties of the change were gnome-control-center's font and theme code, and nautilus-media. Since I wrote the font browser code in gnome-control-center, I updated it to work again. It isn't clear whether nautilus-media will be updated, since the view was a major component of it, and most of the remaining functionality is provided by totem.
If you are looking at updating a Nautilus context menu to use the new extension interface, fontilus-context-menu.c is a pretty good example to model your code on.
One of the big differences is the way Nautilus extensions are loaded compared to the old context menu API. With the old API, you would provide a Bonobo component and set a number of properties in the bonobo-activation server file listing a menu label, the list of mime types the context menu applies to, what URI schemes it supports and whether it supports multiple files. Nautilus could then do a single bonobo-activation query to find out what context menu items correspond to the current selection, and add them to the menu. If the user selected one of the items, the corresponding component would be activated, and an event sent to its Bonobo::EventListener interface.
In contrast, Nautilus extensions are initialised on Nautilus startup. They indicate that they provide context menu items by implementing the NautilusMenuProvider interface. When the user brings up the context menu, the get_file_items method will be called on all extensions that implement that interface. A list of NautilusFileInfo objects is passed in, and the method returns a list of NautilusMenuItem objects. Also, Nautilus extensions are run in-process while Bonobo components could be written for in-process or out of process use.
One of the benefits of this system is the added control of when to display a menu item, and what to use as the label. If you want to only display your context menu item when 42 text/html files and one image/png file are selected you can. However it does mean that each new extension causes some code to be run before popping up a context menu. I have no idea how this compares time wise to the time taken for the previous bonobo-activation query though.
The interface for property pages is quite similar to the context menu interface. As with context menus, you have an imperative NautilusPropertyPageProvider::get_pages interface rather than a declaritive interface based on activation properties. This has the benefit that you can simply not provide the page when the properties in question are not available for the file (with the old setup, you'd end up providing a properties page stating that there is nothing to display).
The other interesting parts of the extension interface is the NautilusInfoProvider interface that lets you attach extra information to files, such as extra emblems or custom attributes, and NautilusColumnProvider, which lets you provide additional columns for the list view that map to custom file attributes. One example of this is nautilus-vcs, which can show revision numbers for files in CVS working copies and adds emblems indicating the file state.
Of course, there are downsides to the extension interface too — since extensions are always in process, they can crash Nautilus or leak memory. However, it was already possible for Bonobo based extensions to do this if they were designed as in-process components and badly written ...
Another issue is that language bindings might find it more difficult to support the extension interface where the language runtime would have to cooperate with Nautilus, compared to out of process Bonobo components where they have more control. I guess we'll see what happens.