Funny, I think Lan isn't really a big issue, he's just a little bit more emotional than he was in the books but he's otherwise a pretty convincing Warder. But they just gave him shit scenes in this season because of all the bickering with depowered Moiraine that wasn't in the books..
For me the main casting issue is Perrin, he looks like a dumb mouthbreather. Hell if it wasn't for Giant Mogatoo Loial around him in the scenes, you'd think the dude playing Perrin is playing an ogier because he looks distractingly out of place. Also not really sure why they gave him the angst of killing his own wife accidently, it's not like Book Perrin needed reasons to be emo even without that.