Seinfeld Is Unfunny: It would probably be seen as a very tame book by today's standards, but the frank and open depictions of puberty (although common for Judy Blume books) were seen as controversial and perhaps even obscene in the 1970s when the book was published.
Some Anvils Need to Be Dropped: Laura is not a bad person for having large breasts. It's just something about her that she can't help, and people should not be treating her badly or creepily leering at her over it. This would be important enough if she were a grown woman, but she's only a young girl.
Squick: Laura at one point complains about male teachers staring at her breasts. The target audience (and indeed, Laura and Margaret themselves) are likely too young to realize the full implications of this. Adults, however...
Norman. He's a genuinely nice (if a bit nerdy and awkward) guy who has a very sweet crush on Margaret, but she wants nothing to do with him, partially because he's so unpopular (she is a preteen girl, after all).
There is also Laura Danker, who deals with boys bothering her over her breast size and the other girls envying her, which leads to nasty gossip about her being a slut. She is just an average 12 year old who attends mass and confession regularly.