Inb4 "Teemo sux remove him," I'd just like to preface this with the fact that Teemo is in no way overpowered, but he is rather underpowered relatively speaking and that is he's hated so much that reflects a design flaw, not a balance problem. Either way, seeing as he is the way he is right now, I'd like to mention some balance issues.
So without further ado...
Ability Power ratio on impact reduced to 0.3 from 0.4
This nerf was likely in response to the new Malady buff that allows it to scale with AP. But... Well, you're nerfing Teemo under several assumptions.
1) He will get Malady. If he skips Malady that just makes him clearly weaker than he used to be. In my opinion, it's bad gameplay design to force a champion to build a certain item/path.
2) Malady will be the first item Teemo gets. Teemo needs to rush Malady for power comparable to the pre-nerf numbers (admittedly slightly stronger but still) during his early game.
3) Malady is affordable. And it isn't. And this is the biggest problem with the nerf. Teemo is weaker until he builds an item. The availability of an item is not a reason to nerf a champion. That means Teemo has to spend 2,000 gold just to be similarly viable. That's a very very evident nerf. Either Malady needs to be changed, the interaction has to be changed, or Toxic Shot has to be changed but you can't just nerf a champion's early game like that.
4) Teemo's Toxic Shot is overpowered. Orianna's passive skill gives her 20% AP scaling on her attacks that rises to 28% with just 2 hits. 2% shy of Teemo's new AP Scaling. If I were to compare Teemo with Malady + Nashor's (my personal core build) to Orianna with Malady + Nashor's, I'd probably like Orianna more! And she'd have more burst damage to boot! I'd also like to note that with Malady's nerfed base damage, Teemo does less damage overall. Especially since Attackspeed is costly now.
I'm going to ignore the shrooms (that's hated anyway) but talk about On-Hit Teemo. Deathcap got nerfed as well so on-hit AP Teemo suffered as it is but the nerf to his scaling hurts him on multiple counts until he builds a Malady. And by then, he's already put off other items that he might have preferred! I used to run Guinsoo Rageblade prenerf on a good game to snowball up some more. But now, it seems like I have to go Malady first or I risk losing midgame viability...