XAML markup that worked in the stable version does not work in preview as the title says. Here is the basic example:
...
<TabControl TabStripPlacement="Left" Height="300">
<TabControl.ContentTemplate>
<DataTemplate>
<Grid>
<ContentPresenter Content="{Binding}" Width="230" Margin="20" />
</Grid>
<!-- Uncomment this line and text will be visible! -->
<!--<ContentPresenter Content="{Binding}" Width="230" Margin="20" />-->
</DataTemplate>
</TabControl.ContentTemplate>
...

Here if I wrap ContentPresenter in a Grid or other elements it won't be displayed when the tab is selected. Removing any wrappers and making ContentPresenter direct child of DataTemplate solves the problem, but the no custom content template layout is possible.