Personally what I think is fun in a video game is a difficult challenge that rewards the player for exploration and careful planning. If am I successful, I feel accomplished and that gives me a reason to move forward. A really good example of this type of game is Dark Souls and the entire Souls series. The creators of Dark Souls, Bandai Namco, created an experience that has realistic design characteristics in a fantasy setting. Players who rely on button-mashing will generally fail due to a combination enemy placement, the layout of the map, and the physics of the weapons. For example, if your weapon is a great sword that is very heavy, you can't swing it quickly or carry it easily. Button mashing with that weapon will not be effective. Dark Souls is not a normal, hack and slash RPG. It requires patience and timing. There are very dangerous scenarios that if you just run in without knowing the layout or enemy placement you will be properly punished by your hasty actions. This game is really great at giving hints about your goals and what you need to do to progress. One example of this is that you have to light four torches in a dense swamp. No one tells you about this goal until you explore the area a lot more. I think the designers of Dark Souls intentionally rewarded exploration and discovery because if you don't explore, you will never finish the game.
If I had a choice in the development in Dark Souls, I would probably add even more secrets like hidden walls and rooms. This would promote more exploration and make the character more aware of his or her surroundings. I would also fix the mechanics of dual wielding. There are two of the enemies, Yorm the Giant and a dragon that are both are based on gimmicks. I would like to change those bosses so they are more challenging and give you more of a sense of accomplishment.