<Window x:Class="SliderTest.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation";
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml";
Title="Window1" Height="300" Width="300" Background="LightBlue">
<Grid>
<TextBox Height="24" HorizontalAlignment="Left" Margin="30,19,0,0"
Name="textBox1" VerticalAlignment="Top" Width="72"
Text="{Binding ElementName=slider1, Path=Value}"/>
<Slider Height="22" Margin="28,64,20,0" Name="slider1" VerticalAlignment="Top" />
</Grid>
</Window>

この書き方は糞
Text="{Binding ElementName=slider1, Path=Value}"/>

こうなるのが正解。
Text="{Binding slider1.Value}"/>