OK, from what I've gathered, the argument is about how good Harper is, and whether or not he can fail. Fact: He is already very good, especially for his age. At the AFL, far above his age level, he put up a .410 OBP, and I can't remember his SLG or OPS, but I know they were excellent. That was in limited ABs. There was a problem, though. He struck out 31% of his official at-bats. Yes, he was very good, and I'm probabaly nitpicking on this- but the fact is, that's a serious problem. Yes, he was in a much older age group, I get it. Do I believe he'll get over it? Sure. He's in a very good position right now, and he's right on track. But can he fail... of course he can. He'd already have to be a star in the Pros for that not to be an issue. Do I think he'll fail? No, I have no reason to believe that- he's more than exceeded expectations. But there is always the chance he'll bust. Always.
Well, this is naturally true. Nothing is guaranteed in life. I think saying someone is a can't-miss prospect is basically saying that you would bet a significant portion of your wealth that he would pan out without losing any sleep at night and if someone gave me even-money on Harper being a star in the majors (whatever metric would be used to determine that) within the next 5 years I would be willing to wager a pretty hefty sum of money on that with little concern.
EDIT: Just to add to that (bear with me as I'm up way too late and just got done watching all of Season 1 of How I Met Your Mother, that show is too damn addictive) I could not see a scenario barring major injury where Harper is not at a minimum an average MLB starter. If that is his absolute floor then I think he should be considered a can't-miss prospect.