Immersion. If I'm just going through the motions, e.g. having to grind 10 levels, I'm not going to have any fun, and the point of a game is for you to enjoy it.
Of course, what's immersive for some won't be for others. I like free exploration, and being able to wander round and find lots of little quirks and talk to people without having the creator's plot breathe down my neck. Others will prefer something they can rush through, meaning Gym Leaders that can be challenged immediately without grinding. Others will enjoy raising their Pokémon more than anything else, so include IV analyser gadgets and biasers (things that encourage certain IVs/natures/moves, etc.). Others will want nothing more than a way to battle online against other players, so just a minimal region is needed. And so on.
No game will satisfy everyone, so make sure you know which gamer base you're aiming for. Importantly, make sure you tell the gamers what's in your game. If you have an epic story on rails, let us know that's the point of your game so that someone like me won't expect much of what I prefer (free roaming). And no, you won't be able to fit everything in. You just won't. Not if it's ever going to be released.