Look, there is no doubt about the sterile cockpit violation in this case, is there? The cockpit crew was in conversation about many things other than the task at hand.
The record also demonstrated a "cavalier" attitude about the checklist.
Other factors receiving considerable mention were confirmation bias, and the contributing factor of the lone tower controller deciding to do some routine non-pressing work that could have been done at any time. I believe that the Board said that the tower controller was not obligated to watch the CRJ take the active runway, but in essence, he had nothing more important to do. Also, apparently the Comair CRJ actually stopped in an odd spot to get their t/o clearance, and too bad that didn't make the controller take pause and consider that something wasn't right.