I wish the first post was updated to clairfy the constant questions about this, but instead I will just link Bulbapedia
http://bulbapedia.bulbagarden.net/wiki/Meloetta_(Pok%C3%A9mon)
I think Meloetta has great defensive and offensive potential, along with the added boon of being unpredictable due to it's versatality.
As a defensive pokemon, it has the ability to switch it's type to resist switch-ins. Aria forme is weak to dark and bug attacks, whereas Pirouette forme is resistant to dark and bug attacks. While this wastes a turn and requires a certian amount of prediction to use effectively, Gamefreak has blessed it with a secondary effect of 10% chance of sleep, which is doubled by Serene Grace. Meloetta's speed isn't anything special, but with a supplementing nature and EV spread it could hopefully shift formes before the attack, soak it up with the resistance, and proceed to wallop with it's more physical Pirouette forme. On top of that, you might even set the opponent to sleep and give yourself an opportunity to boost or slaughter the unfortunate snoozer. I think Meloetta has the best potential as a defensive-oriented pokemon, though Aria formes nothing-special speed stat is unfortunate and leaves a lot to be desired.
On the offensive, Aria is actually kind of nice to start in. Pirouette has a weakness to pyschic which is very common in Ubers (Though I doubt it'll join Ubers considering the hightened standards) but on top of that switching to Pirouette has a 20% chance to put the opponent to sleep, leaving an opportunity to boost the hell out of Hone Claws with the new Gens sleep mechanics, which following up on that can turn Piroutte into a mighty sweeper despite it's somewhat lackluster movepool and by extension coverage. Stone Edge loves that accuracy boost, and it can eat up any Flying-types to counter your STAB Close-Combat.