Edita Este código deve ilustrar todo o problema:

[XmlInclude(typeof(AThing1))]
public abstract class AThing
{
    public abstract string Name { get; set; }
}

[XmlInclude(typeof(IThing1))]
public interface IThing
{
    string Name { get; set; }
}

public class AThing1 : AThing
{
    public override string Name { get; set; }
}

public class IThing1 : IThing
{
    public string Name { get; set; }
}

List<AThing> aThings = new List<AThing>(new AThing[] { new AThing1() { Name = "Bob" } });
List<IThing> iThings = new List<IThing>(new IThing[] { new IThing1() { Name = "Bob" } });

public void Test()
{
    using (StringWriter sw = new StringWriter())
    {
        XmlSerializer aSerializer = new XmlSerializer(typeof(List<AThing>));
        aSerializer.Serialize(sw, aThings);
        string text = sw.ToString();
    }

    using (StringWriter sw = new StringWriter())
    {
        // This line will throw "Cannot serialize interface IThing.":
        XmlSerializer iSerializer = new XmlSerializer(typeof(List<IThing>));    
        iSerializer.Serialize(sw, iThings);
        string text = sw.ToString();
    }
}

O primeirotext gerado poraSerializer será

<?xml version="1.0" encoding="utf-16"?>
<ArrayOfAThing xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <AThing xsi:type="AThing1">
    <Name>Bob</Name>
  </AThing>
</ArrayOfAThing>

Não vejo por que não possoiSerializer faça isso

<?xml version="1.0" encoding="utf-16"?>
<ArrayOfIThing xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <IThing xsi:type="IThing1">
    <Name>Bob</Name>
  </IThing>
</ArrayOfIThing>

em vez de lançar uma exceçã