DatatypeProperty and AnnotationProperty are not the same, especially in OWL (2) DL. In OWL (2) DL, terms defined in an ontology are separated into classes, datatypes, object properties, datatype properties, annotation properties, instances and data values. An ObjectProperty MUST relate two instances. A DatatypeProperty MUST relate an instance to a data value. Therefore, in OWL (2) DL, you cannot use a DatatypeProperty or an ObjectProperty on a class or a property. This is why there are AnnotationProperties, which can relate any entity (except the reserved ones) to any kind of values (either data value, instance or even class or property). This works in OWL (2) DL because the semantics explicitly says that annotation properties should be simply disregarded in the reasoning process. It is thus a convenient way to add information to classes, properties or ontologies. You should know that in OWL (2) DL, AnnotationProperty, DatatypeProperty and ObjectProperty are pairwise disjoint.

In OWL Full, however, these considerations are not very relevant because you can arbitrarily use a DatatypeProperty or an ObjectProperty on a class or property.