I'm assuming that sun isn't hand pixeled. If it is- GOD, TOO MANY COLORS.

In fact, Gale just reported that this has 959 colors. Normally that's a few too many for pixel art. Oh, and while we're on the subject of... stuff, rivers don't go uphill normally. They're also usually not on the tops of gradual slopes either, but... yeah. Here's a super quick edit:

Other comments:
-rivers aren't actually that blue (although I've seen a few come fairly close), and water reflects light a lot.
-shadows are normally much darker at sunset/sunrise times. as opposed to midday.
-why on earth would there be sand on a green hilly area? unless that's dirt, then it should be colored accordingly.
-instead of having a hundred+colored sky, you could use some of those colors on the hills.
-pillowshading. :/ lightsources are your friends, don't shut them out.
-hills could use some texture of some sort.
Yeah.

EDIT: 4 new posts while writing this. Some things might be repeated now.