Playing games is instinct.
From a young age people play games not just to entertain them selves, but also to improve there survival skills.
The "fun" you gain from games is your brain rewarding you for learning and improving skills.
Finding hidden things, collecting, building stuff , cooking, finding shelter, solving problems and hitting things on the head with a stick, these are just few things that games use to allow people to have fun.
Because our brain rewards us for learning these things we can often find tedious actions fun to do in games, chopping wood is hard and boring work yet people can spend a whole afternoon deforesting a minecraft map.
The more people associate a skill with daily survival the more fun it is in a game, a game about math will appeal to people who realize the value of math and would bore those who don't.
Immersion into a game is important, the more a person believes what thy are doing is real the more fun it is.
Graphics is a very important part of immersion because people trust there eyes more than any of there other senses, it is also the quickest way to immerse some one into a game.
High quality 3d graphics doesn't always mean good Immersion, this is do to the way people understand objects and movement and is similar to the "Uncanny valley" effect.
The more convincing a object is the more we notice strange behaviour.
So to summarize:
You don't need the best graphics for a game to be good, but it helps.
What makes a game fun to play is instinct and different people like different things.
Most hardcore and casual gamers have played a "hidden objects game" at some point.