If you look at the five more deeply (adding Vasquez) and sort them by ascending "right to be excited" level.
5. Middlebrooks - the only true "bust". The lack of patience is something you can work with, but the defense never got better and he never figured out how to cover the plate a bit better. He is also not young anymore. (more like the young side of prime) A .310 OBP would have played with a good glove and frequent light tower shots, but the latter has not materialized. And there is the injury history to boot. Him declining to do the AFL really bothered me.
4. Bradley - the question of his hit tool is too big to ignore. That said, unlike Middlebrooks, he already does something at a true big league all-star level. I still think there is a good chance he can hit enough for his glove to play. It probably won't be in Boston though. In either case, a glove like that means he will play big league baseball for a living in some capacity for a while.
3. Vasquez - if you were grading strictly on "probability" he is #1. This is a tool set that can fit on any major league roster if he did not improve one iota. The upside is limited with his bat, but there is still more and the offensive level he needs to rise up to "above average" is not that far away.
2. Bogaerts - You can't unsee July and August. But he showed considerable promise in a lot of other areas. He needs reps at short, and it helps that Boston's best fielding instructor is at the major league club (Butterfield). He has always been young for the level of baseball he was in, and he always caught up. Last season was tough, but it did not dim his star to me much.
1. Betts - Everything that is true about Bogaerts with regards to youth and figuring stuff out applies here too. He has shown more success at the big league level (in a shorter tour granted), and the athleticism and potential at premium positions puts him at #1.
Are there questions to be asked about development with these guys? Sure. But to me, the bigger issue has been the organization being very rash with the prospects. The decisions between "call him up now" and "gosh we need a veteran presence" seem to be made with too little time and too quickly. While I defended the Drew signing due to the horror show that was 3B, one can't help but notice management being very itchy that Bogaerts was not Alan Trammell 2.0 this very second. You saw the same thing with the management rushing to get Bradley on the opening day lineup over a handful of spring at-bats and then anointing Grady Sizemore's disheveled corpse over Bradley using the same amount of evidence. A similar rush job occurred with Middlebrooks in 2012, almost like the management wanted to pounce on any remotely positive story in such a dreadful season.
Contrast this with how Epstein and Francona worked in new guys and kids. You have to be willing to stand by your scouts and the players they are vouching for. There were few players who looked as unready for any sort of professional baseball gig as Pedroia did in 2006. But they stood by him as he worked things out. Even then, they used Alex Cora to provide some veteran help and whatnot - but that is a far cry from tying to usurp the kid.