I've seen this before and I don't understand. What new did Miller add? He added the POSSIBILITY of something new, but they stripped it away instantly when the writers realized "Dexter can NEVER be a family man -- he's a serial killer!" That's when the made Miller seem like a complete monster for no reason. He offered nothing new to Dexter's character that didn't exist in some fashion before ("Can I have people in my life with the dark passenger?" sort of stuff), he just offered that initial and compelling potential of the fact that MAYBE Dex could be a family man.
Lila became a bitch, but I think you're looking at it from an unfair lens. Before she turned out to be a psychotic arsonist, she was built as a VERY compelling romantic interest to contrast Rita (i.e. -- someone Dexter could somewhat share his addiction with). It was an interesting theme because clearly Lila's addiction was to masking her own issues with the issues of others, which made Dexter the ultimate catch, him being all dark and stormy like he is.
It's all opinion, I just think that season 4 had a lot of potential that it simply did not live up to. Miller went from this fantastic glimpse into the future of Dexter to just a run of the mill bad Dad/evil dude. It was fairly lame to me. Season 2 ran on multiple themes from Dexter finding himself (before that became a common theme), the validity of the code of Harry, and a reasonable situation in which he might get caught/get out of it. Now it's like they keep trying to figure out ways to make things seems crazy for Dex, and likewise, have to come up with just way too fantastical conclusions.
EDIT -- Also, I would say without a shadow of a doubt that the greatest psychological impact on Dexter in the entire course of the show was realizing that he did want his son. And that was in the worst season.