It's pretty easy to see who is a top star (because they have continuous highest-quality output), and who will never deliver anything worthwhile. The trouble is the continuum between those two extremes. This domain is dominated by politics, endless grant applications, bureaucracy. Nobody can really tell where you belong in this continuum, maybe not even you yourself. And I also don't believe that academia is the best…
Well, top stars are obvious a couple of years in their career. Or do you think they are obvious from the beginning?
When this is not obvious, which most of the time it isn't, that's where the tough to judge continuum lies.