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Good reasons
>poor distribution of pokemon in the early game heavily limiting your options
>beginning of casualizing trends such as a 3 pokemon gym leaders/4 pokemon elite four
>removal of certain features, though this is a constant within the series and not specific to Gen 5 - depends on how much you care about said features
Bad reasons
>region being shaped like a circle
>no new fancy gimmick
>ice cream and garbage bag existing
Controversial reasons that depends on your tastes
>new aesthetics and animated sprites
>much more story-focused approach compared to previous games and the contents of the story itself
>no old pokemon available until completing the E4
>with no old pokemon the new pokedex could potentially put you off playing if you cannot find enough pokemon you wish to use as nobody wants to play with pokemon they do not enjoy