Great thread.
Here is my opinions (in no particular order)
1) Chris Jericho circa 2001 (he should have gone up against Austin at Summerslam that year.....not Angle. Angle was too newly turned as a face to go up against a "popular" heel in Austin. Jericho had the crowd. Since Austin injured Jericho's friend Benoit at KOTR, the storyline was there as well).
2) RVD circa 2002: RVD was WAY over. Even a push in 2001 would've been great (although I semi-understand the WWE's reasoning in wanting people to "pay their dues" within the WWE). RVD should have garnered some more "significant" victories, so that he could be taken seriously as a legitimate main-event threat for the future.
3) Kurt Angle circa 2001: After 9/11, the WWE decided to make Angle's character into a joke. The WWE bailed on Angle's face turn (which was developing quite nicely). Bad bad move.
Blame the WWE for completely rushing Angle's face turn that year, and then bailing out when things didn't IMMEDIATELY go their way.
4) Brock Lesnar circa 2002: Lesnar should have been the undisputed champion that appeared on BOTH shows. He should have stayed HEEL (despite if a lot of fans CHEERED him). His first loss should have come at WRESTLEMANIA 19 (perhaps to RVD?).
5) Randy Orton circa August 2004: The WWE should not have turned Orton face the day after Summerslam (especially on that very same night where he cut such an excellent heel promo). Furthermore - the WWE shouldn't have blown their WM MAIN-EVENT at Unforgiven.
6) Triple H circa 2002: I still believe to this day that Triple H could have been HUGE as a face. Instead - the WWE shoved him down our throats, and also involved Stephanie too much (initially when Steph was with Triple H, she contributed to tearing the fans away from Hunter). Triple H's brief feud with the McMahon's was also done poorly.
7) Kane circa 2003: Kane was WAY over when he turned heel (and unmasked), but a 3 month idiotic feud with Shane McMahon soon followed. Kane's re-juvinated character was pissed away.
8) Matt Hardy circa 2003: When Matt Hardy first defected to RAW, he was WAY over as a face. Instead - the WWE (idiotically) turned Matt against Lita and wrecked his heat (and also confused the fans). Matt then became a jobber.
Later on (when the fans didn't care nearly as much about "Mattitude"), they re-united him back with Lita.....and were back to square one.
9) Chris Jericho circa 2004: Jericho was doing tremendous as a heel. Even though he was a mid-card wrestler, he was still getting solid victories over the likes of HBK, Kevin Nash, etc. (and still looked credible).
Since Jericho's face turn however, the WWE have made Jericho look VERY "average". Jericho still has credibility due to his immense popularity, but the guy was a TREMENDOUS heel.
10) D'Lo Brown: A guy who could have EASILY been the next main-stream superstar. D'Lo was WAY over with the fans (face and heel). WWE's mishandling of D'Lo Brown was arguably their biggest act of idiocy of all-time.
11) Steve Austin circa 2001: No way the WWE should have turned Austin "face" at Survivor Series 2001. Austin should have kept his "heeldom" until Wrestlemania-18........where he could've then put over the "top face" of the company in the main-event.
12) Big Show circa 1999: The WWE had a great thing going with Big Show, but then decided to make him Taker's bitch. Ever since that time - Big Show has never really been a "larger than life" type character.