Kill ten pirates. How would you complete that quest? You'd run over to the arrow and use your relevant class moves to kill pirates. Auto-attack is likely to play a part, or some core spell in a caster rotation: mind flay, frostbolt, shadowbolt.
Bring a Gyrochronatom. How would you complete that quest? Well, it depends. Maybe you're an engineer, so you could make it yourself. But for that you'll need iron and gold. If you're a miner, you can get that yourself. Otherwise you'll be at the AH to buy the materials. If you can't make it, you could buy it off the AH or find someone to craft it. Maybe there are none on the AH that day.
Bring 10,000 gold. Now what? You could do dailies for a while. Or you could farm mobs for coin and trash. Maybe you prefer to farm materials, skinning certain mobs. Or you could fly around looking for herbs. If you're a crafter, then you can make something people want. What do people want? Now you have a completely different challenge.
Even a fedex quest can do this. Depending on where your hearth is, the trip can be much faster or slower. Maybe you're a mage and all the cities are a click away. Or you're an engineer and you have mostly-reliable and safe transportation to a couple neutral cities as well as various lakes in Northrend. Even the branch of engineering you pick will make a difference.
Notice how different goals can change the experience? Simply changing what the quest giver asks for will personalize the experience, without needing to write a new quest for every single player.
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