Well, I think the main reason we can't yet compare him to Kasparov is that Garry become world champion at a similar age, but then held the title for many years (a decade and a half depending on which title you count after the Fide/PCA thing). Carlsen will win the title, but let's see how long he holds it for. If, for some unfortunate reason (I hope not of course) he feels too much pressure, gets tired, has some breakdown, and loses the title in 2 or 4 years, then no one will compare him to Kasparov, except as being in very early 20s when winning the World Title.