#2: Unwanted Advice
The thing about fans is that they have opinions, like what's wrong with everything you do, and how to do it better. As an example, "stop sucking so much," is one of the most popular bits of advice I get when I receive feedback on my work and clothes from friends and family. So it's an absolute guarantee that Pottermore will be littered with people screaming at Rowling, trying to improve upon flaws they perceive in the HP universe. Claiming Dumbledore didn't actually die, explaining why Snape is actually Harry's dad, why Hedwig shouldn't have died you cruel bitch. As one example, Rowling has promised that new material will be online explaining why the insane rules behind Quidditch are actually completely logical, following the large number of fans who have come up to her to argue about some of the problems they've observed with the Golden Snitch.
Can you spot them all?
For the record, she is talking about me in general, but not me specifically -- I have never "come up" to her to "argue" about Quidditch. I did once "lunge" at her, to "taste her hair," but she did not complain about that, and we've actually laughed about it at some length since then. That said, whatever her rationale might be, it's completely wrong; Quidditch is completely insane. Proposed fixes and explanations for the curious rules (Rowling has late stage syphilis?) will deface the community muggle boards with alarming regularity; by myself alone, if it comes down to it.
Back when the first few Harry Potter books were released, there were many people who were concerned whether it was appropriate to be promoting dark magic in books that were aimed at children.
"Stay still you little shits."
This kind of moral outrage might seem a bit quaint now, in a world where people are awarded their own television shows after leaking sex tapes, but at the time, some people took it very seriously. And we're about to prove them right. Whether it gets taken over by actual Satanists, or just 4chan kids pretending to be Satanists, whatever community features are available on Pottermore will be immediately corrupted to draw pentagrams, hexagrams and sexagrams. Pictures of slaughtered animals texture mapped to robes? You bet. Druidic slurs about Rowling's heritage? Good golly, yes. (Again by me, if no one else. Have to make use of that Druidism minor any time I get a chance.) Faustian bargains with gullible eight-year-olds? This will definitely happen, and the soulless children who result will terrify parents, educators and exorcists for years to come.
"Honey, do you feel anything when you hug Andrew? I'm getting nothing here. It's like he's less of a person."
Check out more from Bucholz in How To Accidentally Throw A Furry Orgy Using Craigslist and A Da Vinci Code Sequel Review (By Someone Who Skimmed It).