My 6 bans would be: (based on my opinion)

= Too much of everything. Waveclear, mobility, CC chaining, strong single target damage, can destroy most laners, hard to gank... etc..
= Too much AOE damage along with no cost for her spamming spells, too easy to escape (hard to gank). Overall I find her to deal just too much damage and the opposing team needs to put too much attention on "saving a CC for Kat". Just too troublesome..
= I hate tanky champs with gap closers.. specially lock-on gapclosers. Think of your poor adc lol. No but srsly, I just hate Vi. Her % damage, her mobility with Q + R, the fact she can ignore all champions and go straight for your ADC. (I would ban her last though)
= OP in meta. WAY too much damage when he builds full tank. Just rediculous. In SOME games I've seen that he can deal close to the same amount of dmg as the adc while building FULL TANK.
= Having played lots of support lately, I'm telling you Naut support is just too strong. He has too much of everything. Almost no weakness. A meta pick that needs a nerf.
= Too much mobility and a strong initiate for teamfights. He can snowball really strong too. Pretty strong ganking potential as well. I would ban him last since he seems to be picked less lately.