In the new version, dictionaries are serialized (unless specified otherwise) as a document which isn't backwards compatible in some cases, for example this class:

    public class Hamster
    {
        public ObjectId Id { get; private set; }
        public Dictionary<IPAddress, string> Dictionary { get; private set; }
        public Hamster()
        {
            Id = ObjectId.GenerateNewId();
            Dictionary = new Dictionary<IPAddress, string>();
            Dictionary[IPAddress.Parse("8.8.8.8")] = "";
        }
    }

In v1.9 `Dictionary` is serialized just fine as a an array of arrays, but in v2.0 serializing this would throw the following exception:

MongoDB.Bson.BsonSerializationException: Element name '8.8.8.8' is not valid.

This can be solved by adding an attribute:

[BsonDictionaryOptions(Representation = DictionaryRepresentation.ArrayOfArrays)]
public Dictionary<IPAddress, string> Dictionary { get; private set; }

But as far as I know this needs to be done on a case by case basis as there isn't a relevant convention pack