In the current state of the game, Kennen is no longer the beast he used to be, and admittedly, he was too strong before, but now, with all of the past 15 or so champs, he's fallen out of favor.
Before, he used to havea 0.9AP scaling on his Q, and combined with the long range and low cooldown, he was THE master of poke. His auto range, which used to be 575 I think also gave him insane lane dominance as his high base attack speed combined with his W simply out poked and out harassed most champs into recalling early and giving him quick advantages in lane. Furthermore, his 1-5 laning harass softened up his opponents quickly so the instant he reached 6 he could just rush in and one shot his opponent with his ultimate. Those were the days of Kennen dominating his lane because most other kits were unable to deal with him. However, with the arrival of season 3, he has fallen even more behind because of items like Flask, Runic Bulwark, his ratio nerfs and auto range, just don't put him up to par with other mids anymore.
Currently, he is still strong in lane, but somewhat lacking in teamfights.
Now many still think "His ult is OP! It stuns so much and does so much damage!"
However, he only hits 6/10/15 (.4AP) times, up to 240/435/630 (1.2AP total).
This number for an AoE ultimate is relatively low on the scale of AP ultimates,and the stun it provides is at random. Because the stun only procs one at a time and at random, stunning a desired target is very unlikely when there are more than 2 champions in the teamfight as they have time to escape. Not to mention all this on a 120 second cooldown. <-*hint hint*
Without his ult, he is a [relatively] low damage dealing single target AP poke that can't deal with more than 1 enemy at a time.
His role (either an AoE nuker or continuous poke) is fulfilled better by many AP champs. He can sort of do both at the same time, but doesn't excel in any one part. I feel that he needs a slight buff to either his poke or his AoE damage.
Why play Kennen when you can play these:
Xerath: better, longer range line poke that does more damage, has a more reliable stun, and has a kit that can hit multiple targets at once. His ult might be weaker than Kennen's but he definitely has way better poke and can hit multiple champs without using his ult. Laning phase weaker though.
Katarina: Still broken, brb gonna abuse her in soloqeue :P Aside from the fact that Kennen counters her in lane, no point playing Kennen over a Kat.
Zyra: Tons of AoE damage, plants hurt like a truck, high base damage, but shorter range. Still more valuable in teamfights because unlike Kennen, who can zone 1 person, she can zone the WHOLE team.
Gragas: Similar playstyle, still wins lane because of insane Q damage and sustain. No stuns but who needs stuns when you can zone better, poke better, and nuke better without having to dive in?
TF: Didn't really want to include him but did because Kennen and TF both poke. I would say TF has slightly better poke because his go through minions and push the wave, Kennen has more single target damage then TF, but TF is way more useful in teamfights and can CARRY.
Brand: This is the Champ I find most on par with Kennen. They both are strong in lane, squishy, and have AoE ultimates. Both can stun (both sort of inconsistently), but Brand’s ultimate does more consistent damage when combined with his E and W. Kennen can only E in, press R and W for some nice damage, which in the end I find almost the same as Brand’s ult. Brand has a bit more consistent AoE, but is restricted by mana, which is fair tradeoff for Kennen’s single target damage (not talking about ults here).
TL;DR: Outside of lane, he is merely a single target poking champ (that doesn't really hit very hard) with little or no kill potential without his ult and is not on par with current champs.