See also this note regarding the recent change in the roadmap.
The main goal for 1.0 is to have a stable API/ABI, which allows to add common enhancements while maintaining a compatible interface.
The main functional change is the introduction of a single Unified Metadata Container, for metadata of all types, which will replace the three existing container classes ExifData, IptcData and XmpData (#585). Along with that, the Key and Metadatum classes will be re-designed/simplified and names which the compiler recognizes will be introduced for keys of known tags (#457).
Currently we have three different but very similar container, key and metadatum classes, one set for each supported metadata type. This design doesn't allow the addition of metadata of a new type without writing a new set of container, key and metadatum classes. Applications would need to be aware of these new classes before they could use any such newly supported metadata type. Several metadata types are waiting: ICC profiles, PrintIM, "generic image info" (#505) for example.