Kennen can build anything he wants and still massacre people

Tank, crit, AP, AD, you name it. The base damage on all his abilities is so damn high, he doesn't even need items to do damage. If you try to do one trade with him, he'll just stun you in the blink of an eye. If you're hiding behind minions, he'll just zap his way over to you and stun you. If you try to cc him, fight back or run away, too bad! You're already stunned!

Are you trying to hide behind your tank? Well, what a shame! He's stunned, too!

Are you building full magic resist to counter him? Well, what a shame! Now he's building full crit, and still managing to deal half your health in magic damage because his base stats are so high!

Are you an assassin trying to kill him before he can stun everyone? Well, what a shame! His base health midgame is too high for you to kill him before he stuns you and proceeds to kill you with all his high-base-damage abilities!

Is your entire team focusing him down? Well, what a shame! He just used his ult, and all five of you are stunned in literally one second.

Are you somehow still alive after that? Well, congratulations! Now you have the pleasure of trying to run away from his massive movement speed ability, because you have maybe 10-50% health left, and he's still full, because you can't attack while you're STUNNED!

The worst part that people don't realise is that while Kennen is being so damn oppressive in every game, he still has four other teammates. If your whole team is trying to focus such an annoying little gremlin down so he somehow can't 1v5 instakill pentakill everyone, there's still four other teammates that have been completely ignored, and given free reign over the battlefield against five half-health champs that had to put everything they got into stopping the one single obnoxious weasel.