I don't get the hate for Austin's heel turn. I thought it was really well done, myself. Those first few weeks of him destroying the Hardys and whatnot, they made the guy look like a legit monster. The biggest problem with it was that there was no dominant babyface for him to feud with besides Undertaker. Kurt Angle vs Austin in the summer of 2001 was awesome, and it really got Kurt over as a serious performer, but sadly we had the Invasion screw up to deal with at the same time. I agree that Austin siding with McMahon out of the blue was strange, because it was out of character. However, Austin as a heel himself was great.
Now, onto some of the biggest Attitude mistakes...
Well, obviously Rikishi's "I did it for the Rock" makes this list. That heel turn basically killed him, and was so underwhelming. "What, this fat dancing joke of a guy ran over the top babyface in the company and now they're feuding? OK...
I'm gonna avoid stuff like Mae Young and the hand. Those were just stupid storylines, we all know it, every era has them. I'm gonna try to get to the big issues.
I think El Chapo will agree with me when I say that one of the biggest mistakes of Attitude was the Fed's handling of ECW guys, specifically Raven and Tazz. My God, Tazz, what did they do to you? They could have done so much with Tazz, and by his second PPV appearance he was taking on Bossman and Bull Buchanan. I'd have loved to see a Tazz vs Big Show feud where Tazz just completely owned him right before he went to OVW. Can you imagine how much that would have solidified him? Instead, by the summer he was jobbing to Jerry Lawler. That was handled so poorly.
Chyna's IC title run was really terrible, but at least she was over. More than I can say for most of the title holders of today.
Billy Gunn's KOTR 1999 win was a mistake, as well. He was very effective with Road Dogg in the Outlaws, but he was just not singles star material. He's the perennial KOTR winner that went nowhere after his win, right along with Mabel. Back then KOTR was a sign of great things to come, and instead Gunn jobbed to Rock at Summerslam and went right back to the tag division.
I think they really mishandled British Bulldog when he came back in 1999. Granted, he might have been in a really bad way at that point, but come on. Jeans, work boots and his pant legs stuff into his socks? He looked like him sort of Neo-Nazi. Completely the wrong thing to do with him. Then he and the Mean Street Posse got paired up. That was a real head scratcher.
As other have said, Wrestlemania 2000 not having any regular singles matches. I love Mick Foley, but he and Big Show shouldn't have been in main event.
I'm sure there are others, as well. This sticks out the most to me.