(55%) The book has a strange structure. The first 20% or so was Harry's struggles against his non-magic foster family (his aunt and uncle) after his parents died when he was just a baby. This was getting interesting and seemed to set up this struggle to be part of the first plot point, but then Hagrid came along and took him to Hogwarts at about the 25% mark. The first plot point should be the first big change or conflict that sets up the rest of the book, but in Harry Potter the conflict in the first 20% was resolved at the first quarter mark!
The book was slow after that, just going through Harry's introduction into the world of magic. Once arriving at Hogwarts, a new conflict is introduced pitting one house (Harry's Gryffindor) against another (Slytherin, which produced the series Darth Vader character). Certainly some of the students from Slytherin are against Harry, but some of the professors don't like him as well. This new conflict is somewhat interesting, but it hasn't been explained enough yet to make it a true first plot point. The book is more than half-way over, and I still don't know the main conflict in the story.
(Finished) Well, right after I wrote the above, the book picked up on advancing the plot and at the 65% mark we finally found out that the main conflict is stopping Voldemort from getting the Sorcerer's Stone (you know, that thing mentioned in the title!). Things moved quickly after that with the author breezing through the second part (the response) and right into the attack and resolution phases of the story. This part of the book was good and kept my attention.
Overall, the writing was good but not great, as you'd expect from a first-time author. But I think what has made this series so successful for kids is the world of Hogwarts that Rowling has created and the characters who think for themselves and who do important things. And Harry is a very likable character. He doesn't have any super-special skills (except perhaps his ability to ride a broom), but what makes him special is something available to every kid--courage to stand up for himself and his friends.
I think if I read the next book in the series, it won't be for the continuing story but to see how Rowling advances as a writer. I've seen with Homer Hickam (one of my favorite writers) a huge development between his first and second books. This gives me hope that even if my first book isn't very good, I'll have learned so much in the process that my second one could be great.