<p>This question intends to provide a list of solutions to common pitfalls, "gotcha's", or design issues when developing WPF applications. This can also include proper design-patterns as long as there is an explanation as to why it works best. Responses should be voted up or down based on how common the type of issue is. Here are the rules:</p>

<ul>
<li>One response per post. This will clearly give the most common issues the highest ranking.</li>
<li>It would be best to provide the link to the a related post or solution already living somewhere in SO land.</li>
</ul>

Problem
: The major issue I have seen so far is that people start coding in WPF with the winform UI model in mind.
Solution
:
WPF is not WinForms/MFC/Win32
So Forget all the UI side assumptions and norms you have used and learned while developing Windows based UI for last 20+ years.
It is very important to understand the core ideas behind this platform, This link-
Major UI Development Breakthroughs in the new WPF platform
will give an in depth view of WPF. Which lists out the following points. The highlighted ones are my favorite features of this platform.
Advanced Graphics
Drawing Object Model
Rich Application Text
Adaptable UI Layout
Flexible Content Model
Lookless Controls
Data-Driven UI
Consistent Styles
Triggers
Declarative Programming