I agree and disagree, actually. You're right, I wouldn't call them carbon copies because they're clearly different characters with their own problems and teenage drama. But you can't argue that Marley, Jake, Blake, and Kitty aren't meant to step into Rachel, Puck, Finn, and Quinn's shoes so to speak. Obviously, the blatant similarities that are there (along with the things that make them different) are purposeful in order to draw direct lines from old ND to new ND.
I don't know. The only one would could potentially fit into a pseudo-replacement category would be Marley. She clearly got the lead female vocalist role, but that's about it. She definitely hasn't been heralded or shown interest in becoming the "female lead." She actually seems to have this overbearing fairness quality to her that people dub as "Marley Sue" even though it was her who ran into relationship troubles by being indecisive about Ryder or Jake. Neither Ryder nor Jake have ever expressed interest or been coaxed into trying to be the "male lead." Ryder doesn't seem to have his pick of any girl he wants at the school as Finn did. Jake also didn't go the route of being a cheater but instead a very loyal and trustworthy partner in his relationship with Marley. Kitty shares the bitchy trait, but not for any of the same reasons Quinn did -- Kitty does it to serve her own agenda, and just for the sake of being mean whereas Quinn was always a bitch to keep her reputation or being vindictive.
Again, I can see someone who doesn't watch the show (like many of the online magazines that post articles about it) coming to that conclusion based on general "looks" of the characters, but nothing more. I think that's originally how this all got started.
This is basically what we knew before the season began:
Marley - short brunette with a killer voice
Ryder - tall, athletic jock
Jake - bad boy, womanizer
Kitty - bitchy cheerleader
Sure, they did some parallels at the beginning (NYSOM, Womanizer) -- but after the first few episodes with some character development it was fairly clear they wanted to make these characters have their own personalities.