Take it from a seiyuus ignoramus: one of the biggest things that I marked about AKB0048 01 was how Nagisa's voice was far too big for her character, even after the 4-year timeskip. The effect is somewhat similar to what many were observing about Urabe (CV: Ayako Yoshitani). The most common reaction was how the voice was "surprising". Mauser wrote, "[Urabe's] voice actress has a very unique voice, totally atypical for anime girls, but appealing." Karen Iwata's work voice is not quite as unique, but it is equally unexpected for Nagisa.
Mrs Iwata has no roles listed at ANN, which may mean anything. A stage name, for instance. Or an escapee from pornography, like Natsumi Yanase. But unlike the latter, I do not expect Nagisa's voice to be as controversial as Chihiro's was in ef.
UPDATE: Omo tweets the obvious: she is a member of AKB48 group! But of course. For some reason I thought that AKB48 talent was going to voice the "elder generation" or even cameos of "original" AKB48 in "historic recordings". This brings up how I was at a panel with Yoko Ishida and someone asked her if she'd like to do some voicework. She reacted with a fright. Said that seiyuu were far too professional and she could never do it. Well, Mrs. Ishida... Karen Iwata seems succeeding admirably to me, except being somewhat inflexible at the squeaky range, perhaps.
UPDATE: Fencedude (a staff writer at Sea Slugs) twittered a link to official character page, which includes a picture of Karen. She is the only one with a different haircut! Also, her name is spelled as "華怜". It's like "Marina": having a surprise kanji.