I dont have too many ranked Viktor games, but I've played him quite a bit (back when I primarily played AP).
I decided to play Viktor because he's one of those champions with tons of potential (like Hecarim) but are really underappreciated. What I like best is his really powerful lane control with Q, and his decent poke with E. His combo, especially his ult, as insane damage, and I think his ult is the highest damage skill in the game.
But why isn't he good? Mana costs. His Q has been changed so that it moves faster to shield quicker, but he still gets beaten by people who have just a slightly longer range (so that projectile reaches first) or people like Vlad, who have instant damage (his transfusion).
Another thing that kind of limits Viktor is that his Augment (costing $1000) is the only skill in the game in which you actually have to buy it. The only trouble is that it delays big items so much, and not getting it severely hinders you. If there was a way to have a two-part upgrade (400 then 600) or something, it'd be great.
The other thing, is he needs to have more choice with his Augments. I don't really feel like writing each thing out, but it just sucks that you can only choose one, and you can't get another partway through the game. If there was a way to switch, or hell, if you take my suggestion above, to buy one Augment: Power (400 gold) and then the second upgrade Augment Death (600gold) that'd be really, really cool.