Windows Explorer is almost intuitive if you've grown up in the paper world. You have data, which you think of as documents that go in folders -- except folders and documents are a sort of deductive user interface in themselves. You had to know how the filing system worked, what to find, and what to do with what you found. You see a bunch of stuff, but what can you do with it? Right-click it and find out. How did you know to right-click, let alone alter folder or filename attributes? Someone showed you or you took the tutorial at least once.
A user interface shouldn't leave users with questions like "what am I supposed to do?" or "what is this icon for?" or "why/how did I get here?" The UI should answer those questions and guide users through the process.
The inductive user interface
Microsoft coined the term IUI (inductive user interface) to describe a type of interface that solved the problems described above. It fit Web design naturally, years before AJAX as we know it: A workflow was broken out into separate pages, each with the sole purpose of completing a simple, well-defined task. After each task was completed, information would be sent to the server and the next page would be loaded. The one-to-one ratio between a task and a screen became the cornerstone of IUI.
Microsoft's guidelines document spells out specific steps to take when designing using IUI, but the core concepts can be distilled down to a single idea: Each screen should have a single, clear purpose, with everything on the screen supporting that purpose -- including the title of the screen. The only items allowed on the screen that don't directly support the purpose are closely related tasks. This exercise in simplicity is actually difficult, but worth it. Simplicity means happy users.
Hey, that's better. The reason for TurboTax's popularity is that it parses the income tax filing process into discrete, understandable tasks.
If the 1040 tax form is a deductive UI, then TurboTax is an IUI. TurboTax breaks down the tax filing workflow into small, very clear tasks. Each task has a dedicated screen that is clearly labeled, with a short description of what information is needed and why, and often asks the user a direct question. Instead of forcing the user to jump around worksheets and add line 18b on page 2 to line 42c on page 3, the user is guided through the process in a logical, linear way. User interfaces that embrace these guidelines are easy and enjoyable to use, even for doing something like filing your taxes.