1 The game is highly addictive
The game utilizes many of the concepts that turn game play into addiction. The game can have addictive effects. See what makes a game addictive.
2 It's very frustrating
The game uses frustration as the main component to force people to buy things. When a person doesn't buy anything he might get very frustrated as he waits for his buildings and troops to be ready. See Frustration in game design.
3 It wastes a lot of time
The game forces players to keep coming back consistently to check on their new troops, to give new orders to builders and to attack other players. This kind of time based motivation results in wasting a lot of time. See also why Candy Crush is so popular.
4 The game remains at the back of the mind
According to psychologists the brain keeps thinking about unfinished tasks until they are finished. In Clash of Clans there are always uncompleted tasks going on such as buildings being constructed. This results in keeping the game at the back of the person's mind all the time.
5 Paying money is very likely
Under the effect of extreme frustration a person can feel like paying money to get his buildings done fast. Once a person tries this once he might get addicted to fast rewards and so he might keep on paying. See the importance of a reward system in game design.
6 Frustration increases as the person keeps playing
The amount of time needed for buildings to get completed grows exponentially. This increases frustration as the person keeps playing.
7 The social element makes it more addictive
The social element in clash of clans , in addition to the desire to revenge, makes the game more addictive. See why Clash of Clans is so addictive.
8 Missions never end
The game was designed in a way to keep people addicted for very long periods of time. Because the resources needed are very large and the upgrades almost never end a person can remain addicted for months.
9 Clan chat might not be good for kids
Clan chat might not be good for kids since it's not moderated. Kids can get exposed to bad words or even get bullied.
10 The need for resources keep growing
Another addictive element of the game lies in the fact that after each upgrade the amount of resources needed to do the next upgrade grows exponentially. The collection of resources is hard and so a person might waste a lot of time to get an upgrade done.