I play a short list of champions. I like to get good at a few, rather than be bad at many. I can carry a game with all of the ones I play, except Kennen.
He just does no damage. His abilities cost way too much energy. It's very hard to build him since all the good AP items have mana on them. He's just not good. He's very outdated actually.
He has tons of damage. Q - 75% AP Ratio, W activation - 80%, E - 80%, R - 150% if all hit. You also get 25 energy for stunning and 40 for hitting with your E. Above post forgot to add 
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The posts in this thread show poor understanding of the game and especially the meta.
Yes, Kennen's damage is atrociously low. He has high scalings, which is not the same as high damage.
He needs to sacrifice AP for tank stats, due to his reliance on getting in close and having to survive for a long time to output damage. Games do not last long enough for his high AP scalings to come into effect, and he's further crippled by lack of wave clear while sieging or defending. You need to run into the enemy team in order to clear waves, and the ADC will generally last hit over your shurikens.
His entire kit is avoidable and often relies on a misplay by the enemy to pull off. There's too much mobility, CC, and disengage in the game, making it easy for anyone paying attention to stop him before he can get close, and team fights are over too fast, making it hard to wait for the enemy team to use everything; by that time, the fight is often over..
The current state of the game is also heavily focused on burst damage. Kennen has sustained damage, which is not viable. You can 100 to 20 three people in 5 seconds, but most other champions can 100 to 0 you in 2 seconds. You don't have enough time to put out your damage; someone will kill you first, or force you to Zhonya and mobility or step out of your ult, or CC and kill you before you even get in range to ult.
He's not worth playing until the pacing of the game slows down.
