Billy Gunn had a great look and had BIG pops. he was getting his rub from DX. He may have been able to be in the main events for a short term after he won the KOTR. A feud with HHH would have been great back then (because we all know HHH was actually deserving of his status). Bottom line... he could have been a main eventer but got injured and when he returned... he just disappointed fans.
He got "Rikishi'ed" ... Rikishi was over when he 'did it for the Rock' but after he got injured he never found it again. I believe Edge will end up in this same category.
Hardcore Holly isn't worth a paragraph to sum up his lack of wrestling ability and charisma.
Val Venis was killed off by the WWF(E) creatively. Ever since his feud with Kaientai
He could also use some different flashy moves that would set him apart and would make the fans care about him. Maybe a front flip over the top rope and to the floor... just something. He has too much of a basic attack so it doesn't appeal to the casual fan. I personally like his style but I can see a casual fan not being too impressed by him.