Now thats quite interesting - see, I would have put far more emphasis on how a book engages with what is universal in human life, how it examines the world and our place in it, how it transcends its plot and its setting and uses them as a vehicle to say something meaningful about the world. And critically, how well the author makes that meaning rise naturally out of a plot, a setting, and characters that feel real and relevant, rather than just being vehicles for whatever the author has to say.
Oh, and on "otherwise there'd be no place in the world for critics" - there are a good few people about who would argue that in fact, there is absolutely no place in the world for critics.
As for the big read - I don't think any categorisation that puts Hitchiker's Guide together with LOTR and His Dark Materials. There are similarities between the latter two (although, IMHO, Dark Materials is so much better) but Hitchiker's guide is something totally different.