in the second picture, there is a repetition of a little black pattern which takes away from the image, and makes it obvious you've tiled a texture here. there's numerous tutorials around the web on this sort of thing, though I don't know one of the top of my head.
Basically, make it blend in better.
With the sign (same picture), the red on yellow doesn't work very well -- too hard to read it easily. It also looks like the grass is eating the wall on the left side of the door, rather than covering part of it.
I don't like the shading in general, though it is better than just plain colors. Think of how the rooms are lit (add a light in the third, too), and work out how light would affect the room. (like, with the fourth room, the light from outside coming in.
Don't let anything deter you, however. Sometimes criticism can make you less excited about your game, and you seem fairly excited, so don't let that happen. Keep up the good work.