It's different because Coke is chemically addictive, albeit much less than the drugs we commonly think of as addictive.

Even though the caffeine is the same chemical in Coke or other drinks, when you become addicted you also feel a compulsion towards the other circumstances around your consumption, like the exact taste of the drink. It's similar to why people prefer particular brands of cigarettes or beer - the preference gets stronger over time as it's chemically reinforced.

Well, it's not stagnant. The formula is (other than the change to HFCS perhaps) but the marketing, which is the true product, is not. What New Coke taught is that people aren't buying a drink for the flavor, they're buying the physical representation of good marketing and the positive emotions that come along with it. They've never stagnated in that aspect.

They've also innovated in logistics. You can get Coke in places where there's no running water.

They way I see it, Coke's goal is: 1) to make you want a Coke whenever you're thirsty (that's the marketing), and 2) to make it very easy for you to get that Coke (that's the supply chain/logistics part)

To go with the notion that a drink isn't purchased on its taste: Take note that Coke does taste different in certain countries, not the least of which because they often substitute sugars, particularly in Diet Coke. Also note that they even change the name of Diet Coke in many countries; a common one being Coke Light.