This is also another area where C# took a narrow view of language features and implemented the bare minimum needed to get LINQ working. Why not take a general view of type extensions, and support type extensions as a proper feature versus a hack for LINQ? That is, support all members, including static members, for type extensions.

Then C# 5.0 brought Async, a huge feature.

In terms of making the language more powerful I would call those on par with extending out extension methods.

* adding useful hacks to Apple classes:

    UIView FindTitleView (this UINavigationBar bar)
    string GetDeviceName (this UIDevice device)

    string FormatViewCount (this Document doc)
    string FormatCommentDate (this Comment comment)

    string SuggestTitle (this Document doc)
    Uri [] GetResources (this Document doc)

For example:

  public static string AsString(this object me, string defaultValue = null)                                     
  public static int AsInt(this object me, int defaultValue = default(int))                                      
  public static float AsFloat(this object me, float defaultValue = default(float))                              
  public static double AsDouble(this object me, double defaultValue = default(double))                          
  public static Uri AsUri(this object me, Uri defaultValue = null)                                              
  public static bool AsBool(this object me, bool defaultValue = default(bool))

    public enum AnchorFace
    {
        /// <summary>0</summary>
        XLow,
        /// <summary>1</summary>
        XHigh,
        /// <summary>2</summary>
        YLow,
        /// <summary>3</summary>
        YHigh,
        /// <summary>4</summary>
        ZLow,
        /// <summary>5</summary>
        ZHigh
    }

    public static class AnchorFaceHelper
    {
        /// <summary>Normal Vector for the face</summary>
        public static Vector3D GetNormalVector(this AnchorFace self)
        {
            switch (self)
            {
                case AnchorFace.ZLow:
                    return new Vector3D(0, 0, -1);

                case AnchorFace.ZHigh:
                    return new Vector3D(0, 0, 1);

                case AnchorFace.YLow:
                    return new Vector3D(0, -1, 0);

                case AnchorFace.YHigh:
                    return new Vector3D(0, 1, 0);

                case AnchorFace.XLow:
                    return new Vector3D(-1, 0, 0);

                default: // XHigh
                    return new Vector3D(1, 0, 0);
            }
        }
    }

        public static int ToInt(this BaudRate baudRate)
        {
            switch (baudRate)
            {
                case BaudRate.BaudUnsupported:
                    return 0;
                case BaudRate.Baud1200:
                    return 1200;
                case BaudRate.Baud2400:
                    return 2400;
                case BaudRate.Baud4800:
                    return 4800;
                case BaudRate.Baud9600:
                    return 9600;
                case BaudRate.Baud19200:
                    return 19200;
                case BaudRate.Baud38400:
                    return 38400;
                case BaudRate.Baud57600:
                    return 57600;
                case BaudRate.Baud115200:
                    return 115200;
                case BaudRate.Baud230400:
                    return 230400;
                default:
                    throw new ArgumentOutOfRangeException("baudRate");
            }