Here's my input...
Any soap star on MG- Most of the soap stars that appeared on MG obviously didn't want to be there, and lacked the personality and spontaneity needed for MG. Most notably, Kin Shriner on MG 98 and Deidre Hall on MG/HS.
George Peppard- I don't recall his playing ability on PW+, but his attitude was wrong. He decided to use PW+ as a forum to complain about standards and practices, and the treatment he got. Allen was clearly upset abot George getting on his "high horse". Allen tried to get George to "redeem" himself by asking him to explain why he felt the way he did, but George only dug himself a deeper hole. George was never invited back after that week.
John Saxon- When you have to resort to getting a B movie star, you know you're in trouble. John Saxon was on Whew!, and by some accounts, never won a single game for his partner.
Peter Lawford- While Peter was an excellent Password player, when he was on The $20,000 Pyramid in 1979, he was way past his prime and noticably either drunk or stoned. During the game, he talked with a noticable slur, and he could barely stay in his chair. Ironically, his gameplaying ability wasn't too bad, but it still wasn't good enough to get to the winner's circle. I happened to get an email from a contestant that was on that same week, and she said he was so out of it, he actually made an entrance wearing shoes but no socks. Another bit of irony is that in Pyramid's earlier days, Peter was a much sharper player, as evidenced in the clip they sometimes showed of a contestant chasing Peter around the Winner's Circle. He also appeared much more sober in the Cullen Pyramid ep. in the trading circuit.
Charles Nelson Reilly- Funny guy, great MG panelist, but on Super Password he turned to Jell-O when it comes to word games. He was on the second week and was not asked back.