Marty Stu: Win. He's Carmen's dream guy and a Satellite Love Interest who doesn't seem to have any real flaws. When Carmen comes clean about her dishonesty and tells him she can often be a mean, bratty person, he comes clean too. His flaw is that... he used to be fat. Even Carmen seems briefly baffled by this.
Romantic Plot Tumor: Good Lord, Kostos and Eric. They are likable in the first couple of books, but by the end, they've become nigh-insufferable in the way they interact with Lena and Bridget.
It becomes even worse in the sequel book. Lena is thirty years old and still obsessing over Kostos whom she hasn't seen since she was a teenager. At least Eric and Bridget have been living together.
Sequelitis: The first four books are pretty well-liked, but almost anyone who's actually read the sequels prefers to forget them.