There is a fine line when discipline becomes abuse. The defining characteristic between the two often stems from the state of mind a Domme is in when they occur. Symbolically, this serves as a strong representation of dominance vs. bitchiness.
There's a lot of women out there that will throw a fit if they don't get their way. These are usually vanilla couples but the woman finds ways to break her man down over time until his behavior resembles submission. I'm sure you have all seen this before, but I'm also sure that you wouldn't describe these women as "dominant."
I often describe this contrast as "Queen vs. Princess," and I feel this serves as the dividing line between discipline/punishment and abuse. A Queen rules calmly. She may be strict and punish severely for rules infractions, but it is governed by principles. When she is wronged she voices her displeasure and may manifest that displeasure through physical punishment. A Princess is a slave to her emotions. When they erupt she reacts immediately and without restraint. Simple displeasure may appear as anger and hatred until her mood calms.
This has great implications when looking at the D/s dynamic.
The calm Queen exerts dominance. A sub's natural response is submission and order is restored.
The explosive Princess throws tantrums. A sub's natural response to her attacks is defensiveness and fear.
This is not to say that a Domme can't lose her cool, but ideally this is either a special case, isolated incident, or something that has been "built up to" through D/s interaction. If a sub makes a chronic mistake for say, the tenth time and it has something the Domme has actively tried to correct, it is reasonable to assume that her intense disappointment will likely bring about a stronger reaction than if this was the first occurrence of an accidental oversight by the sub. If she is in the act of punishing and releasing some pent up anger over a sub's infractions, for her to lose herself in the moment and build intensity as she goes, this isn't necessarily a bad thing and it will often lead to some of the more intimate and rewarding D/s activities in its aftermath. These activities generally contribute to the overall D/s dynamic of the relationship. While the Domme may be hurting the submissive, her love protects any serious damage from happening on both an emotional and physical level.
When out-of-control emotions take over from the start, the end result is usually damaging to the relationship and these are the cases where abuse can surface.