I think Kat is still too capable, despite the recent nerf. I have played very few games where she hasn't wrecked a team in some manner and I feel her ability to chasedown opponents is too great.
I also find Cassiopeia to be too strong at times, especially in the late game. She can take quite a lot of punishment, her poison deals a substantial amount of damage, and Petrifying Gaze can basically decide a battle without much happening before it.
I think Miss Fortune is too capable. She has a lot of DPS, a decent amount of damage by default, a good kit to support the DPS and damage, and has a good base range. She also has a slow that does a good amount of AoE and DPS. Individually the abilities are fine, but in conjunction they are a little much.
The same could almost be said for Caitlyn, except she's balanced from being squishy and has less DPS. I think her Ace in the Hole Ult should fail if she loses vision of her Target.
I do not find Karthus' Ultimate to be balanced. Unless he has vision of his targets, he shouldn't be able to hit them. Now I think it's fair that his Ult hits the whole opposing team, but not fair that it hits without fail and without vision. I think if he has vision it's fair game and balanced, plus I'd be fine with the Ult dealing more damage if he required vision.
These are my opinions, so please remember just that.