Being a child to a then-weak Carol, she was more than a plot point. Most assumed she was killed, but finally having her mystery solved played into any sympathy built for her since S1, where she had to endure Ed.
You are correct regarding T-Dog, but he was one of the faces of the series--a survivor of two seasons, so his loss was one of the shock deaths. Merle was major in how it was all about drama (at the end of it all) for Daryl, so the shocks mounted pretty high in S3.
Karen and the rest of the ex-Woodbury / new prison population did not matter in the manner of the main cast, but in addition to Hershel's death, the Governor's end was major--a villain for a season and a half, and one of the significant plots from the comics.
Lizzie & Mika: they quickly became strong supporting players--just enough that their arc's dark end mattered for them--not just for reactions/drama in Carol & Tyreese. In fact, Carol & Tyreese are still dealing with the aftermath of "The Grove" well into season 5, and it has not been openly addressed to the rest of the group, so this particular pot may be ready to boil over. Especially with the added weight of Beth's murder.
Season 5 so-far we've had Bob and Beth. We've got a bloated cast but we're more-or-less at par with or "big deaths" for a season. We've got a couple "extras" who could maybe be killed off without hearting future plot lines or anything (Tara, mostly) but overall I think we're good on not having any more major deaths for the rest of the season. And if there is one it'll likely be someone more-or-less unimportant like Tara or maybe Sasha. (and Carol!)
Gimple suggested the season would put the characters through dark/bad times before things would improve, but i've also browsed suggestions that another major character will die before S5 is said and done--possibly as a lead-in to the main arc of S6.
