Update: Forecast, Custom Perspectives, and Quick Open
Since this video was recorded, support for Forecast, Custom Perspectives, and Quick Open has been added to OmniFocus for the Web.
As the name implies, the Forecast Perspective allows you to look into the future. Click the View button in the toolbar to customize the Forecast Perspective. You can choose to show a month or week in the sidebar, display or hide deferred items, and designate a today tag.
While Custom Perspectives can only be created on the Mac, iPhone, and iPad, you can make use of them in OmniFocus for the Web. Click the Perspective button in the toolbar to access all of your perspectives. Perspectives you use frequently can be added to the sidebar by clicking the ✩ beside the name of the perspective.
To use Quick Open, press the O key when not editing text to quickly jump to a Perspective, Folder, Project, or Tag.