For the people who loved the Fire Nation and what it was doing? She'd have been an awesome Fire Lord, at least at first. She grew up being extensively trained by Daddy so it's reasonable to assume that she knows Fire Nation government and politics through-and-through, she's skilled in military strategy, and she's good enough at commanding to get people to follow her, even large groups made up of people older and more experienced than her (as we see with the Dai Li). Still, she is prone to letting her temper get the best of her, she overreacts to things, and it clearly doesn't take that much for her to start cracking-- so while she can obviously handle and accomplish far more than the average teenager, she's far from invincible, and there would be a point at which the pressure would get to be too much, especially if Ozai wasn't there to support her and keep her focused on the task at hand. Also, unless she had time to mature and grow into the role, I can see her growing bored with the more mundane day-to-day tasks of running a government-- she'd want to be out there in the thick of things, even during times when it would be more prudent for a national leader to stay back and control things from the war room rather than the battlefield.