Aesthetic or commercial appeal?

I think part of the problem is that the image feels tilted. Look at the water line at the bottom. Now, I think it might just be how things look from that angle. The water line naturally wouldn't look straight because the land is moving away from you. Regardless, it creates the illusion that the image is tilted. If you try tilting it to "correct" the water line (using the crop tool in Photoshop), the waterfall itself looks like it's tilted too far forward, so you're kinda stuck without doing some more involved warping.

Other than the feeling that it's tilted, that looks like a pretty nice image. So, if that's not what their issue was, I can't imaging what it might be!

I disagree about it being unbalanced compositionally. I think the mountains on the other side do a good job of keeping things together and it follows the rule of thirds almost perfectly. The flow of the mountains from a bit up on the left down into the picture on the right helps to draw your eye towards the waterfall which is the focal point of the image.