What are the defaults for Binding.Mode=Default for WPF controls?

Here's a way to find the Default mode supported by a DP -

.NET Reflector is your friend. With reflector, search for TextBox and look at the source for the static constructor (.cctor()). Here, you will be able to find the code used for registering the TextProperty DP:

TextProperty = DependencyProperty.Register
               (
                   "Text", 
                   typeof(string), 
                   typeof(TextBox), 
                   new FrameworkPropertyMetadata
                   (
                      string.Empty, 
                      FrameworkPropertyMetadataOptions.Journal |
                      FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, 
                      new PropertyChangedCallback(TextBox.OnTextPropertyChanged), 
                      new CoerceValueCallback(TextBox.CoerceText), 
                      true, 
                      UpdateSourceTrigger.LostFocus
                   )
                );

Notice that a parameter is passed to the Register method indicating the default Binding Mode: FrameworkPropertyMetadataOptions.BindsTwoWayByDefault. If you use reflector to look at the registration for TextBlock’s Text DP, you will see that no such value is passed, in which case we assume the binding is one way by default.

Taken from Bea Stollnitz's post : How can I update an explicit binding within a template?

Although having some kind of list of important DP's would be very helpful.

Similar to UpdateSourceTrigger, the default value for the Mode property varies for each property. User-editable properties such as TextBox.Text, ComboBox.Text, MenuItem.IsChecked, etc, have TwoWay as their default Mode value. To figure out if the default is TwoWay, look at the Dependency Property Information section of the property. If it says BindsTwoWayByDefault is set to true, then the default Mode value of the property is TwoWay. To do it programmatically, get the property metadata of the property by calling GetMetadata and then check the boolean value of the BindsTwoWayByDefault property.

Source: https://web.archive.org/web/20100209025938/http://blogs.msdn.com/wpfsdk/archive/2006/10/19/wpf-basic-data-binding-faq.aspx

The safest way would be to always be explicit what kind of binding mode you want from a binding.