The characters are very well developed. One of Rowell's greatest talents lies in creating understandable characters: the motive of pretty much everyone in the story is something the reader can relate to. Of course there are some characters whose actions are inexcusable, like Eleanor's stepfather.
The main character's dynamic was also very well done. It did the thing I care the most about in a romance: it felt real. I can imagine these people being together in real life: they had things in common and they connected on a platonic level before they started being attracted to each other.
By all accounts, this should have been 5-star read. And for a while, it was. What made me drop the rating were the last few chapters. In order to give the reader closure the novel kept going for several pages after the story itself had ended. I feel that in that it fell short. The things that happen in those last pages are mostly things I didn't care to know. I would have wanted more information on Tina's motives and how the situation with Eleanor's stepfather and the rest of her family is resolved. We do know (spoiler) that they are gone, but we don't really know what happened: did Sabrina rebel and leave her husband? Where did they go? (end of spoiler)
In spite of that, "Eleanor & Park" is an awesome book and I definitely recommend it. Though, be warned: it's a lot less lighthearted than Rowell's second YA book "Fangirl", so keep a box of tissues at hand if you choose to read it.