The only thing I don't like about Natsumi is her singing voice. Well... hmm. While I like her singing voice, I think it's too soft, so sometimes her voice doesn't really fit the songs she's singing, like it needs more power. During the Yokohama concert, Natsumi got to sing in Manatsu no Christmas Rose and her voice didn't seem to go well with Mariko, Chisato and Ami's because it was so soft. And they don't even have very powerful voices to begin with (with the exception of Mariko).
So I guess all she needs is to put more power into her voice and not sing so softly.
And another thing! I've noticed that when dancing, she sometimes doesn't have as much energy as some of the other girls. She's a really good dancer, but it's especially noticable during the "Oogoe Diamond" choreography which I can't find a link to at the moment.
Ironically, I can't think of anything else to say about my other favorites.