This is so full of shit. If you want a rich/complex experience with fast response, fat client is always going to be the way to go (well, until bandwidth approaches infinity and central server hardware cost approaches zero).

Anything by Diana Wynne Jones is an absolute must - she does both Sci Fi and Fantasy, and all are very original and started my love for the two genres. I cannot recommend her enough. There's tons out there, probably 20 books or more, and it I would even recommend for adults. Other ones I remember from when I was 9 to 11 are Nicholas Fisk, Anthony Horowitz, which are more sci fi IIRC. There's the Redwall books by Brian Jacques and Deptford Mice by Robin Jarvis - these last two series are quite dark & grim though, so maybe wait till a bit later (I read them when I was 10, but I was always weird like that). Also, you can't really beat Enid Blyton's "Wishing Chair" and "Magic Faraway Tree" books, I believe they're available in an omnibus now. I think someone's already mentioned Susan Cooper's "Dark is Rising" series, and of course the Narnia books.

It's easy, it's a little bit like a Jihad.