Generation 5 is by far the worst. I had to force myself to finish the game. The story that was supposed to be the best aspect, it was actually predictable and it lacked good plot twists like the original RGB. Besides that, the plot was supposed to make pokémon battles look less like cockfighting and more like a sport, but it didn't help at all. The only unique factor about Team Plasma was fake after all. Their name makes no sense by the way!
About the gameplay, you can only battle with your pokémon. There's no interaction. You couldn't take your pokémon off the pokéball. There's no cool features like Contests, PokéAthlon, the Underground, PokéWalker, Poké Amie and Super Training. Of course, you had the stupid Musical where your pokémon jump around while you do nothing. We had the Dream World too, but it was lagged as hell and it was really limited and boring.
Besides that, Battle Subway sucked. Features like BF need something special so they wouldn't become repetitive, Battle Subway failed there. By the way, it makes no sense to bring this feature before the post-game, when you still have a lot to do, instead of after the post-game, when you have almost nothing to do.
Oh and a ice cream pokémon. REALLY?!