At 15 minutes Tim talks about star sizing grids, he says that a column with a Width of 250* is 'defined as a minimum of 250 but takes up the rest of the available space'. As a WPF guy this statement got my attention because in WPF this would be incorrect.
The following is an excerpt from the WPF documentation.
In Extensible Application Markup Language (XAML), star values are expressed as * or 2*. In the first case, the row or column would receive one times the available space; in the second case, the row or column would receive two times the available space, and so on.
So in Tim's example the column would receive 250 times the available space, but since he's set the other two columns to a fixed width of 250, it doesn't really make sense. In fact omitting the 250 and just leaving the * produces the same effect, a column that takes up all available space.
Now I'm pretty sure this hasn't changed in Windows XAML either. Looking at XAML documentation star sizing still seems to work the same way as WPF, feel free to correct me if I'm wrong.