I think the difference is that Wiggins will likely be a rookie of the year candidate despite having areas to work on. Yes, he has areas to work on, but what he has now is far greater than Ennis and Grant. Ennis and Grant will have things to work on if they want to receive playing time, especially in regards to Grant (though I actually have more faith that he'll stick in the league than I do Ennis).
Same goes for guys like Randle, Parker, Embiid, etc. Randle doesn't have a right hand and he's a poor defender, but his left hand, relentlessness, and rebounding ability are great enough that he can work on those other areas while still being a starter from day 1. If Jabari declares, he will immediately average 17-20 points per game. Yes, he needs to improve defensively, but he's ready to start right away. If Embiid can get healthy, with his array of post moves and shot blocking ability, he too will be a starter from day 1.
I think Ennis and Grant both made the right decision for themselves, but they have a long ways to go before they're ready to receive major NBA minutes. Ennis needs to either become a much better shooter, passer, or athlete. As of now, he's probably a below average shooter for an NBA PG, below average athlete, below average defender, below average finisher, and just a solid passer. There really isn't much that he can bring to the table as of this moment.
If Grant can get his back alright, he can bring a high motor and athleticism to the table, but if he doesn't improve his strength or ball-handling, it won't really matter. He needs to get stronger so that he can play as a face-up 4 (like Blake Griffin but obviously not nearly as good), or he needs to become a much better ball-handler and shooter so that he can play on the wing. Otherwise, he too will probably be a bench/D-League player.