Here are some things that make video games fun:
1) Challenging yourself. Like playing a game without using a certain skill, something like playing a game blind(like that The Legend of Zelda: Ocarina of Time player who beat the game blind), or like overcoming a previous record. This is seen a lot in Super Mario Bros. side scrolling games and TMNT: The Manhattan Project. The former has #2 of the list here, too, whereas the latter doesn’t have that(#2) and is more of a challenge.
2) The reward factor. This is getting a reward after completing some game’s phase. Like in The Legend of Zelda it’s getting a new useful item that you could use. In RPG games it’s obtaining a new skill, improving your stats(from grinding etc…) or getting a new item.
3) Finding out new things. I am not sure how big of a deal this one is. In this Dragon Ball game called Dragon Ball: Goku Hishouden it was fun experimenting and finding out new skills by making combos.
4) Doing different combos. Like the Gameboy Advance game Ultimate Muscle: Path to the Superhero, you can do different moves at different times. Also you can do that in DBZ: Goku Hoydenish as mentioned above. It rewards you with positive feelings/emotions unlocking something.
Also Read: TYPES OF RPG GAMES