You're doing a marvelous job! On this style of map, I think the large labels are sufficient to delineate your plains and deserts. Based on the locations of the mountains and water, I have little difficulty seeing where the desert would leave off and transition to grassland.
The coastal glow might not be right for this map--I think you should probably either find a way to make it look less regular, as if it were watercolor, or switch to a different style that looks more hand-drawn. Ghalev's woodcut effect might work well. Wood block stamp style water (S. John Ross' PS tutorial)(+in GIMP)