You guys are acting as if this site is gonna cost money.
IT ISN'T.
The only money spent will be whatever users wish to spend to get e-books of the seven novels and any merch (wands, scarves, whatever other stuff will be in the shop) they like.
The site itself will always be free. Yes, there will be virtual money used within the game, but there is no scheme set up like, say, Farmville where you can spend real money to get extra-special virtual money to get the extra-cool stuff.
No, everything will be earned, for free, by doing things on the site.
When Pottermore goes live, users will be living the books, one chapter at a time. So, for example, the very first "experience" on Pottermore will begin at Privet Drive, where we'll learn how Petunia and Vernon met.
JK has written 18,000 words so far for Pottermore; I believe that is only for Book One. Book Two doesn't premiere on the site until next year.
So as you work your way through the chapters, more and more new information, written by JK herself, will be revealed (almost like having an encyclopedia, but getting to have fun while learning the info!).
When you get to Diagon Alley, you'll get a starter amount of galleons with which to purchase your supplies, including your wand. You'll take a special quiz at Ollivander's--the quiz being written by JK herself--and then be given your specific wand, chosen out of 33,000 different combinations (I believe that's the number I read).
When you get to Hogwarts, you will have your Sorting. Just like if you really were attending Hogwarts, you will only get this one chance to be Sorted. You will answer an extensive quiz (again, written by JK); there are so many questions available, that no one person will get the exact same set of questions (therefore making it impossible to cheat), and none of the question/answer sets are stupidly obvious like "What's your favorite animal? a) Badger b) Eagle c) Lion d) Snake."
There is even the possibility of reaching a "hat stall," which is what happened to Harry (and, we learned from the LeakyCon panel, also happened to McGonagall), where there are two possible Houses for you. So there's the possibility you may actually have some choice concerning your House.
Once sorted, you get to see your common room. The Common Room is where you will be able to chat with other users sorted into your House. You can visit the Great Hall where you'll be able to chat with anyone from any House.
As for the people making remarks about the info about Petunia & Vernon: chill. It's only a single example from the very first "chapter experience."
An example used in the LeakyCon panel was concerning McGonagall. We learned her birthyear (1925) and her blood status (father was Reverend Robert McGonagall, a Presbyterian minister; mother was a witch). We also found out that we will be learning about McGonagall's tragic heartbreak early in life when she fell in love with a Muggle boy.
And these are just very tiny summaries of things we will learn on the site. From everything I've read about Pottermore, it seems we will truly be kept busy in the time between the opening of Pottermore and the release of Chamber of Secrets.
If it ends up that we get one book a year, we Potter fans will have new things for us to do and read for at least another seven years.
And who knows? Maybe by the end of Deathly Hallows on Pottermore, JK will find herself interested in writing about the "next generation," and we can live out Albus Potter's years in Hogwarts via the Pottermore website.*
*this final paragraph is obviously just wishful thinking