There are a few huge disappointments in my opinion, but I will do my top three.
1) WCW Invasion
This is single handedly the biggest disappointment in wrestling history. Ever since the days of WWF and Jim Crockett Promotions fans dreamed of the day the two promotions would cross paths. I remember as a kid PWI and some of the other wrestling magzines would run an issue almost annually centered around dream match scenarios between the two companies. In 2001 it finally happened, Vince McMahon now owned both companies, every major star in the wrestling business was now under one umbrella. Then the "Invasion" happened, and the biggest star we got from WCW was either DDP or Booker T. Instead of Flair, Hogan, Goldberg, Sting, Hall or Nash we got names like Sean O'Haire, Chuck Palumbo, Shane Helms and Lance Storm representing WCW. After all the dust settled WCW and ECW were destroyed by WWE with Booker T. vs. The Rock being the biggest program between the two. If only WWE would have been patient and held out Bishoff, Flair, Hogan, Hall, Nash, Goldberg and Scott Steiner would have all been avaliable for a full fledged "Invasion."
2) ECW
I for one loved the One Night Stand PPVs, I thought it was a good way to relive some of what I loved about ECW. The matches were great, the talent for the most part recieved due respect and maybe another year or two of these events would have been just fine. Unfortunately, these PPVs did so well that WWE decided to make ECW it's own brand. Had it been booked a little better I think the concept could have worked, but this was not the case. The "Originals" aside from RVD became nohing more than jobbers to future stars and guys who never amounted to anything (anybody remember Kevin Thorne and Mike Knox?). Should I even mention December to Dismember?
3) Nexus
I got excited when this angle started. These guys came out of nowhere and started laying a path of destruction. I mean look at their hit list: Cena, Taker, Jericho, Big Show, Edge, Mr. McMahon the biggest names on the roster, all got their asses handed to them by this new young rebellious faction. I thought the concept was awesome, I felt that Darren Young didn't fit in, but aside from that these guys looked great to start off. Then Summer Slam happened. If this angle was ever going to get over, they needed to win at Summer Slam last year. Then they could have had Cena and his crew fight back and beat them at Survivor Series if nothing else, but they needed to win at Summer Slam. If they did lose it needed to be by DQ in my opinion. Getting beat clean made them look weak and beatable. I think Barrett needed a title run, Gabriel and Slater needed the tag belts long before they got them, and Sheffield or Otunga could have got the IC title to make it really look like a hostile takeover. I think getting rid of Young and Tarver was good and adding McGillicutty and Harris, while I think Skip Sheffield getting injured was a huge blow to the faction. I think if the group would have stuck around as a major force, Brodus Clay would have been a nice addition as well.
There are a ton of other disappointments on my list as well. I have a list of stars a mile long. I thought Chris Harris (Braden Walker) was a big disappointment to a lot of hardcore fans. This guy was a part of the biggest tag team in TNA's short history when he signed, and had put on some five star matches with James Storm against every team on TNA's roster. I agree with many in the fact the Goldberg/Lesnar match was a mess and booked pretty awful as well. Turning Austin heel should have been executed a lot better than what it was. Hogan's return and his short title run was a huge disappointment. I think this Raw GM deal is a huge one as well, being that it has been going on for over a year and still no reveal as to who is behind it. One a lot of people forget too is The Rock's last match. The Rock N' Sock Connection vs. Evolution at Mania 20. I always felt that if Rock was going to put any of those guys over it should have been him and Orton on the undercard, and Orton beat him clean. I know there are a lot of Orton haters on these boards, but for Orton's character at the time and for what they wanted to do with him, this would have been huge.