What makes a computer game good and fun

the above answer is not very good
i am currently getting in to game design
iv learnt all the tips and tricks seeing as i am one who comes up with game ideas, writes the storylines etc.

The one key thing to making computer games fun is keeping the player interested.
the recent Call of Duty modern warefare 2 was rated as one of the best games ever created.
the reason for this is the storyline kills the player more than any other game.
throughout the story line the player continuously gets shot and stabbed and betrayed by the same person over and over. then at the very end of the game finally the player gets their revenge by killing the guy who kept causing the players to lose. after the build up of being killed so many times it makes winning the game that much sweeter.

denying the player the opportunity to complete their objectives is one of the major techniques of game creators to create good games

there are other things that make them fun such as:
- build up of tension,
- Reality RPG games can be relative to the player which makes it that much more interesting.
- having locations in the game that are recognisable to players such as: (venice etc)
- speed, for example racing down a mountain having a very short time to react to rocks that may be in the way or trees etc.

hope this helps