Jon posts hist own comparison of MXML and XAML, to which Rob responds...

The CSS debate was a huge one internally, and in the end I think you will find visual triggers, timelines, and visual trees to be a powerful way to change the appearance of controls... One of my big issues with CSS is that it really only works for "already known" customization points. With VisualTrees you can specify arbitrary new appearance using vectors, images, video, text, etc.