Here's my two cents;
Katarina - love most of her lines, and also her voice fits pretty well.. most of the time. There are a few lines, such as her 'intriguing..' and 'time for the danse macabre' that really don't sound like her at all. Especially the later is just said in a way that would fit a middle-aged angry landlady more than a young ninja-assassin-thingy-girl.
Morgana - Whoever is acting her, needs to stop talking from behind gritted teeth. Granted, her character is angry and stuff, but now you just overdid it. More Fallen Angel, less Angry Angel.
Nasus - Pure win. 'Nuff said.
Taric - One of my favorites, too bad that his looks don't match his voice at all. Ditch the shield and age him by some thirty years, and we're talking. The soft, tranquil we that he speaks in is totally cool and fitting for his character. Totally awesome lines, too.
Fiddlesticks - Fiddle is one of my favorite hero concepts. I like his voice okay and all, but he needs more lines. Seriously.
Rammus - So.. this guy is supposed to be known for his wit and twisted humor. So.. um.. yeah. Grrr, mrrrgrg, grmgrgrrr, and all that translates into smart jokes in armadillo language? Okay, carry on, then..
Evelynn - Okay, whoever did her voice, give her a cookie. Now. Her voice fits her character perfectly, the lines are good, and just from her voice, you can tell she's a naughty, naughty girl.
Mundo - I really don't like this character, although the concept of a mad doctor is good. I think Riot didn't do as well here as they could have, but Mundo's voice sounds exactly like it should for this type of guy - big, mean Darwin kid who just wants to decorate the christmas tree with your kidneys.
Singed - I think that his unelaborate, plain lines fit the character well. However, his voice sounds like two different guys were making it, and Riot couldn't decide which one was better, and just put them both in the game.
Anivia - She sounds just as benevolent as she should. Although there are a few strange bits that I don't think match her character - 'in a flash' and 'keep cool.' Very good work otherwise, I must say.
Zilean - Pretty good, not much to say here. His taunt makes me laugh aloud every time I hear it. Hah.
Cho'Gath - He's a big, mean creature with even worse attitude. In other words, his voice is just the right one for him.
Kassadin - I like him. Not much to say here.
Ryze - His voice and line seem both to impatient and eager for such an experienced wizard, no matter how primal his magic is.
Teemo - OH GODS PLEASE, MAKE IT STOP, MAKE IT STOOOOOOP!
Gangplank - He's a pirate. Of course his awesome. Yaarrrrr!