1: It has to be addictive. Factors of this are the other no.1 and no.2, but also, believeability, good graphics(not always though), and just fun gameplay
1: The story has to be good. I like long in depth stories for RPG's, and shorter but really interesting stories that give a motivation behind the FPS
2: You have to have interesting characters. If the character is boring, your story is boring, and the player has no motivation, and thus no reason to play the game.
3: There has to be some challenge. Make it hard to get where i need to go,but it can't be so hard that the player can't get farther, this inhibits the storyline and makes a boring game. There are exceptions sometimes, if the difficulty is parrt of the storyline. Another thing is don't make it take too long to get from place to place. Make some shortcuts!(think town scroll from diablo2 or the portals in sacred). Also, in larger scale games, i like having more than one way to solve a puzzle, complete a level, and kill the guy. A good example is Hitman 2. Also, the level design has to be smooth. Don't make it a maze unless its some dungeon in an RPG. Another good thing is to include a map, bu i think you can only do that in commercial or pro. But if you have them, make use of that.
4: You have to control the player easily, its no fun if it has impossible button configurations, like a,y,m, and insert as controls. the most popular is the w,a,s,d where w is up, s is down, w is strafe left, and d is strafe right. the mouse is used to make turns and look.
5: Originality! Don't copmletely clone another game. If it's a simialar plot, thats ok, but make it a different game. You know, different weapons, places, enemies, your on the other side, stuff like that if you have to copy another game. However, it is best not to copy a game, or a tv show, because once that game or show is no longer the popular, your game will not be played.
6: Detail! I like levels that have some detail to them. Don't have bare floors, walls, or such. No-one is that clean, lol. I guess the only exception would be a hospital. You also need to break up the level somewhat. You can't have box rooms with only enemies in them, have some obstacles.
In the case of a game on addicting games is that it has to have some kind of purpose, it has to be really bloody, and it has to be able to be played indefinitely.
Wow, this is my longest post. I'm kinda proud of myself