Mental Models
When introducing a new application, it's important to help users in their transition to this new world. Although I agree with Cameron's take that when designing applications it's best to begin with the core actions, remember that your users will probably be presented something else before these core actions (i.e. dashboard).
This is where mental models come in. I'm finding myself thinking and talking more about these lately. Simply put, a mental model is an explanation of how something works in the real world. This is what will help users transition to the new application.
The problem is I look at new applications and say, "I don't get it. How does it work for me?" Now, I understand that I may not always be the target audience, but when it's a "for everyone" application, make sure you have a clear mental model for the potential users. Don't make them think.
Of course, the best solution for presenting a clear mental model is designing it right. The UI and interaction should do the explaining. The Apple Calculator, Page Layout view of Microsoft Word, Google Homepage, Stickies, Writeroom, and the Finder/Folder are some examples that I like. (Writeroom is probably my favorite since there is no UI. You open the application the first time, and you have a blank screen with a blinking cursor telling you to start typing.)
There are also other ways to supplement mental models: 37signals' designing for blank slate state, a screencast or video introduction, and a simple infographic.
I'm not saying to design and build your application wholly around a mental model—you need to design for the first time/infrequent use and also the everyday use. But mental models can be very helpful for your users' transition to a new application.