Note: I played the yellow version, but this is really a critique of the red, blue, and yellow version combined since they are all basically the same game, just with different monsters.

-Lots of Pokemon monsters to collect (gotta catch 'em all). Throughout the game all of the different people in the Pokemon world are obsessed with collecting all the Pokemon monsters like it's the thing to do, and the player psychologically agrees!

-Sometimes Prof. Oak will encourage you to do your best and collect more monsters. This encourages the player to catch 'em all.

-Each Pokemon monster has it's own unique abilities, appearance, and sound that it makes. This adds variety and depth to the game.

-In some games you will spend time collecting abilities, weapons, items, and other things. Collecting a Pokemon monster is more gratifying because each one has it's own element type and a unique set of abilities that can also be improved. The player gets more things for every item he collects.

-A very simple, easy-to-learn battle system that minimizes the learning curve. A short learning curve means that the player can spend less time trying to figure out how the game works, and more time having fun.

-If the player doesn't collect some Pokemon in a certain area as they are progressing through the game, they can always go back to get them. In some other games, sometimes you have to pick up items at a certain time in a certain dungeon. If you miss it, then you will have to go through the game again to get it. Pokemon eliminates this problem.

-There are clues that tell the player where to find some of the Pokemon that they don't have. This minimizes that need for an FAQ, and also minimizes player frustration.

-Aside from the people in the Pokemon world encouraging the player to collect Pokemon, the Pokedex also does this. If the player sees a blank space in the Pokedex, then he will naturally fill the need to fill that space. It is another device that the game uses to encourage the player to catch 'em all.

-The next objective is never too mysterious, as it can often be in RPG's. The player almost always has a good idea about what to do next.