A few things in my mind>#1 POKEMON
it is the name of the game... literally, having a wide variety of pokemon that not only are readily available to promote choices, but have them actually reliable, which is one of the few negatives vs gen 2 and HGSS which has the reliable mons be gen 1 mons like pidgey, while having new mons like ledyba/spinirak having little to no usability, which even beedrill/butterfree had some. As well as a negative for gen 5, which was a large portion were blocked off until a decent portion of the game is through and you would hopefully have a full team by then anyway, which was helped a bit in BW2, but that is my opinion, and are not limited to these gens only, but the 2 that i recall the most at the time.
The mixture of challenge, fun, and exploration that makes things not boring, and entices you to keep playing. such as the constant cutscenes from SM make the experience boring until you get those few moments you can explore an island unhampered, it ruins the flow of the game and fun of it as a consequence.
Something that is engaging, but isn't hamfisted every 5 seconds of the game, something that allows you to do your own thing, but when ready, you go to continue the story, not railroaded into the story all the time.