I see this in two areas -

1): My buttons have stopped changing colors when you put your cursor over them. I define my button like this:

<Style BasedOn="{StaticResource Style.ButtonBase}" TargetType="Button">
<Setter Property="Background" Value="{StaticResource Button.Background.Static}" />
<Setter Property="BorderThickness" Value="3" />
<Setter Property="BorderBrush" Value="#355C7D" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Border
Padding="{TemplateBinding Padding}"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<ContentPresenter
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"

                        TextElement.FontSize="{TemplateBinding FontSize}" />
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Style.Triggers>
        <Trigger Property="IsMouseOver" Value="True">
            <Setter Property="Background" Value="{StaticResource Button.Background.Hovered}" />
        </Trigger>
        <Trigger Property="IsPressed" Value="True">
            <Setter Property="Background" Value="{StaticResource Button.Background.Pressed}" />
        </Trigger>
        <Trigger Property="IsEnabled" Value="False">
            <Setter Property="Background" Value="{StaticResource Button.Background.Disabled}" />
            <Setter Property="BorderBrush" Value="{StaticResource Button.Background.Disabled}" />
        </Trigger>
    </Style.Triggers>
</Style>

2): I also use MoseOver directly in some controls, and they have stopped working as well:

    protected override bool ConnectEvent(object source, string eventName, string handlerName)
    {
        if(eventName == "MouseEnter" && handlerName == "Status_MouseEnter")
        {
            ((FrameworkElement)source).MouseEnter += Status_MouseEnter;
            return true;
        }

        return false;
    }

private void Status_MouseEnter(object sender, MouseEventArgs args)
{
// Is not invoked.
}

The only change has been updating to 3.0.4; everything works correctly on 3.0.3.

edit: I don't get any errors or crashes, so I'm not sure how to reproduce this.