I'm a little confused, I do have trees placed throughout, which denotes "variety" to me. So I may need something a little more specific to know what you mean by that.
He means don't put trees in a straight row, as it looks very artificial. If you want to have a more natural looking screen, make sure that there aren't any long straight lines. This applies to both trees and mountains (something I forgot to mention in my post about your mountain screen). Consider this screen that Shane posted earlier in this thread:
Notice how often the mountains curve. This gives the area a more natural feel than a straight path. Speaking of paths, the one in your shot looks very artificial as well. Try to give it some more variety by making breaks in it, and having it not quite reach the treeline in some spots. This time, a shot from Demonlink:
This isn't quite the best example, as you're trying to make a forest path rather than a ruin, but it still illustrates the point to a degree. If he had just used the, well, "tile" tile for the whole path, it wouldn't look as good, because it wouldn't give the screen much variance.
I think I should probably clear this up; I'm not trying to insult your screens or your ability to make them. I seem to remember reading a screen design tutorial by Moosh (at least, I'm fairly certain it was Moosh) that showcased a method that involve "cutting" the screen into nine pieces like a tic-tac-toe board, and making sure that each section had something going on in it. I couldn't find the thread, so if someone knows what I'm talking about and can find it, please post a link to it.