您忘了命名空间在XML声明：

You forgot about namespace declared in your xml:

var xdoc = XDocument.Load("buildings.kml");
XNamespace kml = "http://www.opengis.net/kml/2.2";
var buildings = xdoc.Root.Elements(kml + "Document")
                    .Select(d => new Building {
                        BuildingName = (string)d.Element(kml + "name")
                    }).ToList();