Slack (lack of tension) is definitely the enemy, but I think targeting slack as the number 1 problem is cheating from a corrective point of view. Reason: its too broad. Slack can creep into casts in many different ways, each of which requires a different solution mechanically. Slack can occur before the lift, in the lift, in the set, in the pause while the D forms, and possibly in the forward cast itself. Identifying "slack" as the single-most important problem is not as bad as saying that the most common problem is poor casting, but it's probably a close second in terms of discreet corrective solutions.