Mundo got a pretty nice buff last patch. Buffed his cleaver health cost by making it refund half cost on hit, meaning you can't spam it (since it takes out a pretty large chunk), but if you're good at hitting you can still do alright. His ult can also do a base HP regen of 70% at level 3 now (I think it goes 30%/50%/70%). Few other small buffs.
Health regen Mundo is arguably one of my favorite characters to play, even though it's not too viable in my ELO. But it's pretty hysterical to have a base 300+ hp/5, so you can run from an enemy for a few seconds and have almost full HP again. Unfortunately, since Mundo isn't a "tank" per se (he has nothing that makes someone want to attack him), a tank build doesn't work too well with him, especially when you can get someone like rammus or shen or cho for around the same cost. DPS build sucks on him, since he doesn't really do all that much more damage than a tank build (Thanks to Masochism), but has a SERIOUS case of glass-jaw.
Katarina's ult isn't too bad. Give her a stun/snare/knock-up/knock-back/silence, etc, and she's left in the middle of 5 people with no way out. Same with NuNu, but his will do no damage
. Bouncing blades is her real killer, I've seen them do 1000+ base damage before, even the last hit would hit for some 500 or so. If you ran next to a creep wave after a team fight it would wreck you. If they scaled that down some, despite her being a complete face-roll character, I think she would be pretty balanced. But she's not really OP unless you are playing a champ that relies on high HP regen, like Mundo or Cho.
Nasus is actually really balanced. Pretty good farmer and pusher, but not an OP pusher/farmer like Sivr or Taric, excellent 1v1 (he can down any AS carry like Ashe, Xin, Shaco, Trist, etc), and a great team figher (with his AoE Ult, small AoE damage with an armor debuff, wither to destroy the enemy carry, and Siphon Strike which just gains power every time you use it (especially when they are affected by Spirit Fire)).
But he's far from OP. ;P
None.