As much as it leaves a vile taste in the mouth to do so, I think we have to admit that Zuko does have a lot of power potential. Power alone, however, is meaningless without direction. Which Zuko, incidently, lacks.
An interesting question that I suppose belongs on this thread is whether Zuko will ever be capable of creating lightning. I really don't think he ever will be. Spiritual peace is not enough - if it was, anybody could learn the technique. Iroh said in 209 that only "a select few firebenders" have the ability to do it, leading me to think that just as not every person is a bender, not every firebender can make lightning.
Also of note is 306, in which Iroh informs Zuko that:
"Good and evil will always be at war inside of you Zuko. It is your nature. Your legacy".
We learn here that Zuko is always going to be a fairly conflicted kind of guy, suggesting to me that lightning bending is out for him.