Your area does not have to satisfy all four categories but, to be most effective, you should have at least one category of player in mind when you make your area. Consider your audience and maybe solicit input from members of that audience as you work in order to fine tune its value to that group of people.Do your quests satisfy only achievers in that they provide a desired result after some work? (e.g. collect x amount of whatever ears)
Do they also satisfy explorers by spreading goal out over a complex and diverse environment? (e.g. the ears are found in only certain spots of a large area)
Do they also satisfy socializers by adding some opportunity (or requirement) to interact with NPCs and other PCs? (e.g. the quest requires multiple classes of PC or a long talk with Link, the retired elf fighter who lives in your area)
Do they satisfy killers by culminating or including some great battle or maybe equip the PC for future battles? (e.g. the victorious quester finds a +18 sword or must battle the fearsome shoggoth in order to escape with his or her bag of ears)
I hope that this is useful!