Pokemon Let’s Go Pikachu and Eevee are being created to cater to wider, more casual audience than the main series games have ever aimed for before- this includes a lot of simplification of series mechanics, such as catching wild Pokemon.
Even apart from those, there seem to be a lot of changes in these games- for instance, your Pokemon now follow you, you automatically ride Pokemon, your starter Pokemon is always by your side, and so on. One of the more significant changes, however, may just have been posted on the Japanese Pokemon website (translated by Siliconera). It states that gyms will now have specific requirements players must meet before they can challenge them, from having certain types of Pokemon on your team to meeting a certain level requirement.
No doubt this change has also been made to ensure that those who do challenge gyms can actually beat them- but on the other hand, it also seems to take way from the joy of creating and customizing your own party, and taking on impossible challenges in game with it. I cannot imagine that this change will go down well with many long time players.
Pokemon Let’s Go Pikachu and Eevee launch exclusively on the Nintendo Switch on November 16.