Let us say for the moment that all "if you ... then I will" statements constitute an implied threat of force (leora's insightful bit on ultimata notwithstanding). Let us say also that some of these are legitimate and others are not (the difference between "coercion" and "pressure"). One thing I have just thought of that might delineate the two is the set of initial conditions for social interaction that preceded the ultimatum. In many relationships, it is clear at the outset what power each individual may legitimately exercise within the relationship. For instance, bosses are allowed to ask their salaried employees to work extra, unpaid hours in order to get caught up on an important project. This is part of what it means to be salaried. On the other hand, bosses are not allowed to do this with hourly employees. In that case, "Stay late or I'll fire you" would be grounds to haul The Man into court for keeping you down.

The trouble is that many, maybe most, human relationships do not come with a set of initial conditions that everyone agrees upon. Romantic relationships are an obvious example of this (and maybe this is part of why most people screw at least one of them up in some preventable way). Friendships can be another - basically anything where power is supposed to be evenly distributed or where the balance of power is not universally understood. Even in parenting the lines aren't clear anymore as our views of children evolve; "Do as I say or I'll beat you" used to be thought of as not at all coercive (in the pejorative sense) but as good parenting, but now in many circles it's strongly frowned upon.