I think the main component which justifies the use of the term "metro" is that all of the public transport systems in the area, be it Newport Bus, Cardiff Bus, all Rail lines, and any light tram systems, will all be called the metro, and will all use the same ticket system, making it possible for you to travel anywhere in SE Wales on any form of public transport using one ticket, or card.
This, probably alongside services which are promised to be up to twice as frequent will make this one awesome project.

One thing though, if services really are going to be that much more frequent, Cardiff Central is going to be a nightmare.