I liked personally some of the new features in .NET 4.0 and just want to share those.
The new String..IsNullOrWhiteSpace method indicates whether a string is null, empty, or consists only of white-space characters.
New Path.Combine method overloads enable you to combine file paths.
You can now enable lazy initialization for any custom type by wrapping the type inside a System.Lazy<(Of )>) class.
The compression algorithms for the System.IO.Compression.DeflateStream and System.IO.Compression.GZipStream classes have improved so that data that is already compressed is no longer inflated. Also, the 4-gigabyte size restriction for compressing streams has been removed.