Ok, I think I have to stick a note of sanity into this... Just to say at the outset, I'm a fan of Terry Pratchett, but I love his books neither more or less than JK's. First, his letter happened to be printed in the Sunday newspaper, not his fault it happens to be JK's birthday.
I just read the Time article, and I have to say, I see why Pratchett is angry. The writer of that article does seem to think that in order to praise JK they have to insult all other fantasy authors. Did you read the article? Seriously, was it just 'Insult CS Lewis Day'? I love fantasy, and it really is quite aggravating to see someone generalising an entire genre as "deeply conservative--politically, culturally, psychologically". Is that because they tend to be based in a world without "cars, telephones and PlayStations"? That's the point of books, to give us views into other worlds, not to all be based in this time!
Anyway, Pratchett was more giving out about that article and it's insults of the rest of the genre than JK herself. He's right, the world of fantasy is much more than a place "where knights and ladies morris-dance to Greensleeves". Maybe JK isn't a huge fan of fantasy, but that's no reason for the writer to insult some of the greatest authors of the last century.
This is becoming a complicated rant so I'm just leave off here and let you continue your insults...