I actually never imagined Stannis to be imposing. Rather grim, sullen and stubborn. But even then I agree, so far the the HBO-version lacks seriousness and strength. But his voice I like, actually.
Wait a bit and we'll see.
Might not be what you were referring to but I noticed a little change in overall demeanor as well, it seems like they really let loose and had a little fun with the dialogue -- particularly a couple lines Tyrion says and other instances with other characters as well (Maester Luwin had a little moment and there was another one that I forgot). I think it's kind of cool and shows writing maturity.
The biggest scene that erked me the most was Rob's scene with Jaime. They both did a good job for the most part but the last bit with the digital wolf felt incredibly contrived. It was like, yeah.. okay.. we get the point: Rob is a badass but the wolf scaring Jaime was a dumb and unnecessary exercise of their apparently new CGI budget.
Also, for as good and award-winning of a job that the actor who plays Tyrion does, his glaringly fake accent bothers me a lot