I believe the selling point to Pottermore is JKR's new information for the fans about the Harry Potter universe.
I've read the Harry Potter novels since I was in primary school, following them release after release since the first book. I did similarly with the movies. I administrate a Harry Potter roleplaying forum. I'm familiar with the canon and the universe as it has been revealed to date.
It's the new information that's intriguing. McGonagall's backstory was brilliant to read and really gave new insight to who the character is. It's things like that which I most look forward to. Not pretty pictures. Not collecting random items. Not brewing potions.
Frankly, the implementation of potions and duelling seem like that of a simple flash game which anybody could have created to play on any number of platforms.
If I wanted a Harry Potter video game, I'd buy the video games.
I'm interested in the gold mine of information that exists about the Harry Potter universe but has yet to be made public.