I  assume since most of the games here are horror games, then shock value or gore    will be your number one answer.

But for me,   I would say great music, hidden secrets,  cool effects,   a good battle system,  interesting story   and intriguing characters.

It's hard to say cause it depends highly on risk vs reward the game offers. If the risk is too much compared to reward, the game became not fun and too hard but OTOH if the risk is too few compared to reward, the game became boring and not fun. I didn't say exactly what risk and reward would be cause each genre has its own risk n reward.

For action games, the reward is feedback and physical reward from successfully "connecting" the action and the risk is the obstacle and hurdles obstructing player from performing the action.

What is "fun" also depends on your audience.    What is fun for little children is going to be different than what is fun for adults.   My game audience is universal, rated G for general population.   So I don't want any offensive content in it.  

Fun to me is a puzzle to solve. Something that you don't see right away, but once you do the pieces all click into place. It's deeply satisfying. I actually hate horror games because of how little mental effort is required. Make an FPS, turn the lighting down really low, throw in a jump scare and there's your game. Seriously, you'll wear out your adrenals like that. 

For me, the answer is a very technical concept that many developers overlook...
and overlooking it has caused even many AAA games to fail: pacing.

It’s the speed or rhythm at which the action, narrative, and mechanics unfold.

The alternation between tension and calm is what determines whether a player gets bored or keeps playing.

Interesting how no one is really saying graphics :)   Yet if you hear mainstream games it's almost all they prioritize imo.

for a MMO i would say good community and have somehing to show off and to do like fighting creatures and go on a quest

/tags

Horror 110k, Pixel Art 234k, Adventure 208k, Puzzle 174k, Action 244k, Platformer 212k, 

Horror is not the most on the platform. But horror usually dominates the popular pages, because of all the youtubers driving traffic to Itch.

What makes a game fun to play

What makes a movie fun to watch or music fun to hear? Games are interactive media, so there is another dimension to it, where the thing can be appealing. Strictly, visuals and audio are a bonus for that other dimension. There are pure text games. Games without sound. Pixel art. The most successful ever game is Minecraft. And that is as ugly as it gets in regards to high res graphics.

Mainstream games are nice to look at, because it is expected. They have the budget and no reason to create low res games. Except if they try to appeal to the pixel art fans or try to imitate Minecraft.

So, what does make a game fun to play? Depends on the genre. What makes a driving simulator game fun to play, will not be the same as will make a platformer game fun to play and differ yet again from what will make a management simulation game fun to play and differ yet again from what makes a puzzle game fun to play.

Fundamentally, you need to entertain the player. Good music and good graphics can be entertaining. There is even a /games/tag-walking-simulator genre, so some nice scenery is to be expected there as a feature. But there are just so many things that will be entertaining for different people in different ways, that there is no specific answer to your question.