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If we're talking about disappointments, I think Gamefreak severely mishandled the implementation of the Fairy type.
Very few new Fairies were added, with only a handful of unique type combos. And despite Gamefreak themselves saying that they wanted Fairies to counter Dragons, only a handful of Fairy types can actually stand up to even the weakest Dragon types.
I was certain that they'd at least have a pseudo be Fairy-typed, but instead we only got Goodra (not that Goodra isn't cool).
Only a few older Pokemon were retconned into Fairies, and it seemed mostly arbitrary. I still can't think of a reason why Mr. Mime is a Fairy. Meanwhile, Wigglytuff and Clefable become Fairies, yet Blissey's line is cut out. Disregarding how that would impact the metagame, why isn't she a Fairy? She's a round, pink, Normal type and is even in the Fairy (!) egg group, just like those two. Hell, Audino fits those criteria as well. Furthermore, I don't think anyone would have complained if Azelf, Uxie, Mesprit and Cresselia became Fairies.
Fairy also doesn't have a lot of moves, with only Play Rough, Moonblast, and Dazzling Gleam being worth using at all. To compound the problem, Fairy moves aren't very widely distributed.
I think the Fairy-type has a lot of potential, but Gamefreak needs to explore the possibilities of the type. The definition of what exactly makes a "Fairy" is pretty vague, so there is a lot of room to make really interesting concepts for Fairy Pokemon and moves. I just hope they get expanded on in the future.