The main problem with the "Pokemon" games is that they've followed the same repetitive structure for 20 years. Any changes made to the formula before now have generally felt marginal from a casual perspective, which made it difficult for me to care for several years.
"Pokemon Sun & Moon" shake things up for the first time, completely uprooting the design in favor of something better.
You see, in the old games, you would travel from town to town, leveling up your team and taking on each town's Gym Leader. When you conquered a gym, you would get a badge that would let you take on the next gym.
This time around, we're in the new, Hawaii-inspired region of Alola, a chain of tropical islands. Each island has a set of unique, element-based trials to complete, with each trial eventually rewarding you with access to powerful, spectacular super moves for your Pokemon.
Once you finish all the trials on a given island, you take on that island's Kahuna, who essentially replace Gym Leaders in Alola. Each Kahuna has a varied team of powerful Pokemon, so you'll need to keep a well-balanced team yourself.
The new trial-based setup brings a ton of much-needed variety to the series. In one of them, you have to watch Pokemon dance and memorize their moves, while in another, you navigate a haunted grocery store on a ghost hunt.
Each trial culminates in a battle with a Totem Pokemon, which is a beefed up version of a regular Pokemon. These battles can be tough, as Totem Pokemon can use unique abilities and call in allies to ruin your day.
Overall, it's several orders of magnitude more engaging than the old Gym system because there is so much variety. Not every trial is amazing, but the good ones far outweigh the bad ones.