It's a tricky thing to define, but I'll take a crack at it. A design is addiction-based to the degree that it encourages players to experience the same content again and again (often referred to as grinding) in return to obtain a series of rewards. These can be simple labels with no tangible effect (like an in-game title or some achievements), or they can be character improvements that give the ability to move on to a new location with a slightly different sort of grinding. I call this the grind/reward cycle, and it can keep players coming back to one game for years.
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Similarly, in Lego Star Wars (and other games in the series), practically everything you do, from killing a foe to smashing open a door, results in your receiving studs (money) for buying rewards and characters. No matter what you are doing, the illusion of accomplishment is maintained.

Addiction-based design isn't a boolean, zero or one, true or false type of thing. Practically all successful games provide satisfying rewards to a player in return for long and skillful play - like unlocking new areas, making a stronger character, getting more stars in Guitar Hero. But some designs are built entirely around taking advantage players of being vulnerable to this sort of positive reinforcement. And, in the case of World of Warcraft, I am extremely susceptible... me and millions of others, which is why I now fearfully avoid it.​