I've been working on a pet project recently and been deeply buried in the configuration of System.Diagnostics. Whenever I find myself writing configuration code at the moment my heart angsts to be back playing in a land filled with Xaml. Here's a sample of configuration for System.Diagnostics today:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <system.diagnostics>
        <sources>
        <source name="MySource" switchValue="All">
            <listeners>
                <add name="myListener" />
            </listeners>
        </source>
        <sharedListeners>
            <add type="Ukadc.Diagnostics.Listeners.InMemoryTraceListener, Ukadc.Diagnostics" name="myListener">
                <filter type="Ukadc.Diagnostics.Filters.MultiFilter, Ukadc.Diagnostics" initializeData="some, nasty, string, full, of, settings"/>
            </add>
        </sharedListeners>
    </system.diagnostics>
</configuration>

Now imagine a world of configuration marked up in Xaml...

<?xml version="1.0" encoding="utf-8" ?>
<Configuration xmlns="http://schemas.microsoft.com/netfx/2009/config"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:src="clr-namespace:Ukadc.Diagnostics;assembly=Ukadc.Diagnostics">
    <Configuration.Resources>
        <src:InMemoryTraceListener x:Key="myListener">
            <src:InMemoryTraceListener.Filters>
                <src:MultiFilter Setting1="StronglyTyped" Setting2="Values" />
            </src:InMemoryTraceListener.Filters>
        </src:InMemoryTraceListener>
    </Configuration.Resources>
    <Diagnostics>
        <Diagnostics.Sources>
            <TraceSource x:Name="MySource" SwitchValue="All" Listeners="{StaticResource myListener}" />
        <Diagnostics.Sources>
    <Diagnostics>
</Configuration>

Type checking, extensibility... oh the joy. Totally invented on the train home, but it would make me a very happy man.