Two of the core tenets of web design is "don't make users think" and "manage to focus users' attention" (see great list at Smashing Magazine). In short - this means that every pixel should be aimed at conveying what is most important and driving users to the right behaviors. This is a difficult task that some excel at... and it is made even more difficult by the mobile web - which provides far less real estate and different user interactions. Below is a screenshot of Quora's iPhone site (*not* an application despite looking like one). It is notable because it perfectly embodies Quora's core components and user interactions:
1. Notifications: the Quora logo dynamically displays the number of notifications you have. For a web component, this is very nifty - but it is very useful and displays the speed at which Quora's content is growing / moving.
2. Search box: Big search box with grayed out call to action. On the core web site, this is the major call to action.
3. User ID using Facebook Connect - which is the registration mechanism.
4. A series of major buttons that are the primary calls to action: ask a question, notifications and inbox. All revolve around the core question & answer premise.
5. Rather than view on your notifications, you can view Quora's entire log of changes (questions, answers, tags, edits, etc).
6. Invitations: Quora is still invite-only and invitations are an important role in user growth and virality.