This has been bothering me for a while, maybe I am missing something.
The following throws an error with the commented attribute (expected >), but shouldn’t I be able to do something like this?
<Label x:Name="Gaga"
FontSize="20"
<!--
Content="{Binding SomethingThatIsEmptyAtDesignTime"}
-->
Content="LookAtMe!"
/>
Though you can’t comment out using the basic XAML markup, you can achieve the desired results by importing the Open XML markup namespace.
This blog post describes how to do it.