If anyone should have been bigger in their days it should have been Jake Roberts or Owen Hart. I know they were arguably bigger than anyone else on that list, but those are two guys that should have been a lot larger than they were and it's a crime they were left off the list.
The main one on that list I completely agree with is British Bulldog. That guy was awesome and even though he had a few flashes of stardom, most-notably his match with Bret at Wembley Stadium, WWE could have done so much more with him and he had everything you could possibly ask for in a superstar.
And just to touch on a few others the list mentioned:
Raven - Don't misunderstand me. I'm a fan. He was absolutely one of the greatest mic workers the business ever saw and if WWE took him as seriously as Paul Heyman did in ECW, I have no doubts he could have risen to Mick Foley's level. I've found that some of his work gets overrated at times because he often tried way too hard to be "dark" and came off very silly at times, but he had Vince McMahon to steer him like he steered Vince Russo, Raven could have been a very big deal. His WCW work is notable of course and he was always stuck in a similar situation that many other mid-carders were, particularly Chris Jericho, and he was never going to be a big shot there. That being said, given the opportunity in WWE, I really think the guy would have been compared to Jeff Hardy in the long run. His substance abuse problems, bad attitude, and seemingly nihilistic lifestyle would have eventually become a serious problem for any promoter who put too many eggs in his basket. I respect the guy for what he's done and he was always a great talent, but I don't think he ever took the business seriously enough to warrant being any bigger than he was and I think people saw that in him.
Chris Candido - Very similar situation as Raven's. Although I definitely don't think Candido was world champion material, I think if his bad habits and demons were kept in check better and he hadn't came to WWE during such a weird period of that company's history, he might have received a better opportunity than the Bodydonnas gave him. Getting involved with XPW and their porn-funded promotion was probably the worst mistake of his career though. No one walked away from that company and went on to do anything special and that was a very poor choice by him and Tammy.
Bam Bam Bigelow - The guy headlined a Wrestlemania, so I'm not sure why he's even on this list. The guy had his shot.
Mike Awesome - Excellent physical specimen, but not a good wrestler and very average at best on the mic. He would have just been another Bobby Lashley in the wrestling world and that's really not a compliment. I think his constant involvement in hardcore matches throughout his stay in ECW really shaped people's perception of him as having far more potential than he really had. Take all the weapons, tables, and other foreign objects away and you're left with a severely-replaceable wrestler.
Hakushi - Japanese? No, Vince doesn't like their type. "Could he have been bigger?", perhaps, but it wouldn't have happened in WWE. Let's be very clear about that.