Some design ideas are good ideas simply because they improve usability. Here are some common ways to improve usability with Expression Blend and the Microsoft .NET Framework.
Model the real world
You can use custom visuals and interactions to make specific controls look and behave like their real-world counterparts. This technique is best used when users are familiar with the real-world object, and the real-world approach is the best, most efficient way to perform the task. For example, simple utilities like calculators just work better when they model their real-world counterparts.
Show instead of explain
You can use animations and transitions to show relationships, causes, and effects. This technique is best used to provide information that would otherwise require text to explain what users might miss. For example, a book for young children could animate page turns to show how the controls work.
Improve affordance
Affordance is a property of an object that suggests how the object is used (instead of using a label to explain it). You can use custom control visuals and animations to suggest how nonstandard controls are used.
Use natural mapping
Natural mapping is a clear relationship between what the user wants to do and how to do it. You can use custom appearances and interactions to create natural mappings when standard common controls won’t do.
Reduce required knowledge
You can use custom interactions to limit the number of ways to perform an operation and the amount of knowledge required to perform a task.
Improve feedback
You can use custom control visuals and animations to give feedback that shows when the user is doing something correctly or incorrectly, or to show progress. For example, the Address bar in Internet Explorer in Windows Vista and Windows 7 shows the progress of loading the page in the background.
Make objects easier to interact with
A model of human movement known as Fitts’ law states that the effort required to click a target is proportional to its distance and inversely proportional to its size. For example, you can use animations to make objects larger when the pointer is nearby and smaller when the pointer is far away. Doing so makes the objects easier to click. It also lets you use screen space more efficiently, by typically making objects smaller.
Focus
You can use rich layout and custom visuals to emphasize screen elements that are required for the task, and to de-emphasize secondary elements.
Additional resources
If you are designing for Windows, consider adhering to the Windows User Experience Interaction Guidelines to establish a high-quality, consistent baseline for all Windows-based applications, regardless of how they are implemented.