I can agree with you, dept of field does not look quite realistic but can help. Do you know how when your eye sigh is really close to an object and it blurs the background? For example, when you are reading a paper pretty close, the background blurs. It is possible to make it something like that but can be extremely annoying to players and may not fit your game.
It looks great. Consider adding a little more variety into the blending between sand and grass when you start to do the detail.
Also, i would add some randomness to the edge of the raised area, currently the ridge it too uniform and sharp. Take some chunks out of it in places, etc.
Also maybe make the ridge not quite so vertical unless its supposed to be manmade and reinforced with concrete etc, natural cliffs will have a slight slope and variation (even if theyve been landscaped by humans).