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The beginning of the downfall at Game Freak.
They decided to go with the baffling decision that, after Pokémon Platinum, which brought the most new ideas and content out of any of the games until then, they wanted to retread generation 1 yet again, but this time, it wasn't a remake, per se; they would redesign every single, and that means EVERY single generation 1 mon into a weird generation 5 mon, some for better, and some for worse.
As if having a very samey roster wasn't bad enough, it also insisted on creating some kind of grandiose story, and brought up a lot of questions concerning key concepts of the games, such as Poke Balls, and how they work. And that isn't a problem, mind you, as long as it's the final entry in the series. It turns out they planned to ask these questions, ignore them without giving them a complete answer, and then pretend they never happened in the future games.
The direction behind G5 is a nightmare to look over. An absolute trainwreck.
That being said, however, I am one of those guys who's now about to say how BW2 is one of their favourite gamesl GF somehow spring-boarded off the terrible disasters that were BW1's direction into one of the most content packed, unique and well put together games in the series.
Generation 5 also made my favourite Pokémon line, the Beheeyem family, so while there's that, it also created such happy accidents as 98 base speed Hydreigon and the genies. The trash mon isn't bad, and while the ice cream mon is flawed, it's because of bad evolution design rather than the theme itself. It just gets bigger.
tl;dr, mixed bag, but I'm glad it happened.