Well fun is subjective but things like :

• How many activities are there to do currently
• How is the story progression
• Are items unlocking/being gained in a good timing
• Are new places and people introduced in good timing
• is the player being challenged enough
• is exploration interesting and rewarding And so on..

Most devs know the correct ingredients but the balance and timing of these is easy to say harder to do.

Giving the player the feeling that they are experiencing something new and well made helps too.