I just installed 2012.1 and having 2 problems with AnimatedExpander. Firstly, It is ignoring HorizontalAlignment="Center" in the header. My XAML looks like this(simplified):
<shared:AnimatedExpander ExpandDirection="Right" >
<shared:AnimatedExpander.Header >
<AccessText Text="_1. Book" HorizontalAlignment="Center" />
</shared:AnimatedExpander.Header>
The header text does not render "center" but instead "left" which apperars at the bottom since the expansion direction is "Right". The Control that this is in is fairly tall, so the text is far from the arrow button.
This is componded by the second problem: The background color is the same as the control. This makes the button and text look like two separate things. Only when you hover do you get a contrasting color.
I tried unsuccessfully to work around the second problem. Suppling Background color on the header only happens to the text. Supplying on the control,changes the background for the entire content.