There is a large body of usability research available on standard web interface, but recent UI advances such as AJAX have many sites breaking new ground. The field is in need of new standards to help users over their learning curves.
A more interactive web will improve usability when functions related better to people's
existing mental models. For example, Google Maps allows users to grab and pull the map to
scroll in two dimensions. This is an intuitive model because it's similar to how we use
real-world paper maps, and it's clearly more usable than separate scroll buttons.
Other mapping sites quickly adopted the click-and-drag navigation metaphor, but many
common interactive behaviors have no standard. Millions of pieces of user-generated content, such as photos uploaded to Flickr or blog posts to TypePad, are created every day, and even using keyword search it's a challenge for users to figure out what content is worth their time. However, there is no standard method of rating user-created content. Digg articles may accumulate "up" ratings, but other sites give 1-5 stars, and still others, like Wikipedia, eschew ratings altogether.
Social networking sites have no standard visual metaphors to describe the rich interactions they make possible. Typically, it's all or nothing: either you're connected to a person and "in" their network, or you're "out." Filters to show and hide content among subsets of your network tend to be text-only and awkward.
The winners in the Web 2.0 space will shake out in part due to the success of their interfaces. Watch market leaders like YouTube and emerging sites like Eons to ensure your company chooses familiar design metaphors that become standards.