The problem with it is that it looks fake, "photoshopped" would be the precise term. When you're doing landscapes you have to aim for that painted look (unless its abstract, but even then you need to be painterly), even in cell shaded animé shows the backgrounds are always painted.
Fix the water so it looks like water and not a mirror, you know, with ripples and stuff (hint, lensflares aren't physical objects, so wouldn't be reflected in water ;P). Get rid of the horribly pixellated black lines on the mountains, then get creative with low opacity brushes. I assume it's photoshop... Also slap some trees innere.