Both JK and Terry Pratchett offer nothing new to the fantasy genre but make the most money. However, JK's young readers will increase the readership of fantasy.
Wow, Lace, that's absolute rubbish! It's almost like the good
old days.
JK Rowling, while her books have made a lot of teens read more (whether they keep on reading is another matter), it's true what other posters are saying, she's simply regurgitating an old and worn theme, hardly adding anything new. In fact, I'd be so bold to say she's simply dumbed down the popular classical fantasy story arcs (which are quite dull to begin with) and made it readable for a generation which is barely literate. Getting them to read is pretty much the only achievement which I'd laud her for. It's treating the symptom, not the cause.
Pratchett, on the other hand, earns his living in the borderlands of fantasy writing, providing a social commentary and a reading pleasure not a lot of writers can match. And for me, that's what fiction is for, whether plain old fiction, sci-fi or fantasy, they should provide a new perspective on things we might not see clearly in the muddled backdrop of our everyday life, or issues which are otherwise too sensible to dissect and analyze (and perhaps even solve). Sure, you might enjoy the odd simply entertaining book once in a while, but the ones that stick with you are the ones that do this. And through the Discworld series Pratchett does exactly this. Although in a very unsubtle, but effective fashion.
Getting someone into Pratchett might be a tad more difficult than getting them to succumb to the Harry Potter bug, but it's an addiction that will almost certainly stick and take you deep down the rabbit hole. JK Rowling leaves you waiting in Kansas.