Note: Before you read this post, it is using the public consumer preview (beta) of Windows 8, VS 11 & .NET 4.5, so I expect some issues will likely be resolved in later releases. Check the site out for more info!
With the beta of Win8, VS 11 & .NET 4.5 now out I thought I should post again (first post about this can be found here – recommended reading to see how it has improved) how it has improved or changed since the alpha. This is not meant to be an exhaustive list, it is a list of the most common things (where most common is what I use, because I am pretty common 
)
Namespaces
Namespaces have been polished and there is a much better alignment of the new awesomeness to the old so this is getting much better.
#if NETFX_CORE
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml;
using Windows.UI.Core;
using Windows.UI.Xaml.Controls.Primitives;
#else
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
#endif
Duplication of INotifyPropertyChanged, ICommand & INotifyCollectionChanged is SOLVED!
I mentioned about the EPIC FAIL of the duplication of core interfaces – that has been solved! 
ObservableCollection<T> is broken is SOLVED!
The double facepalm that was breaking ObservableCollection<T> has also been solved – so this means your Metro style apps are more like your WPF & Silverlight apps than ever before.
User Controls must be created on the main thread is SOLVED!
I did not get a stupid behaviour where a user control had to be created on the main thread, and thankfully that has been solved! You can now create user controls on other threads!
IValueConverter has been changed
Previously the Convert & ConvertBack methods second parameter was a string, now it has been changed to a Type. This is a good move as it allows for better compares, but means any IValueConverters from alpha will be broken and it is a simple change:
//Before broken public object Convert(object value, string typeName, object parameter, string language) //After changing type of second parameter = working public object Convert(object value, Type typeName, object parameter, string language)
Good work image from http://bloggers.com/posts/happy-developer-37379