(Note: some of the things I say in this post got Ninj'd back when I started writing this)

Well, this is doesn't look bad, I can see that you actually put effort into it :)
But there's some thing you could improve:

1) The overall colors looks a bit off, the grass should be a bit more blue-ish and the dirt should be a bit more desaturated
2) The grass should take more space (this might sound a bit ridiculus but trust me, it does look better most of the time)
3) You should add more shading, it really does make the tileset look better
4) The tiles are a bit repetitve; you shouldn't just make one single 16x16 tile of the grass, it looks un-natural
5) Your tileset is a bit too ''pixelated'' if y'know what I mean, that's pretty much why I don't make tiles directly 16x16 :P
If you want to get rid of that effect without having to make a 480x480 tile and then resize it (like I do <_<), I would recommand to never use plain color, shade with more colors and apply gradients more often

Also, don't get me wrong, I don't think your tileset is garbage, it's actually pretty decent :P
I just wanted to share some of my experience I had with tileset design