Haley's critical error this offseason was making the team - more importantly, the receivers - adjust to his gameplan instead of gameplanning around the talent he already had at his disposal.
I agree with this statement 100%. Many of the key players that are, currently, on the Steelers offense were brought while Bruce Arians was the Offensive Coordinator, to suit the needs of his Spread-Offense. When Todd Haley was hired, he let it be known that he was going to adjust his offense to fit the players that he had. It is apparent that he did not do a good job of it.