Well you're definatly heading in the right direction. Since I'm focusing on buildings I'm not going to mention the grass or other details.
Try to stick with the same type of roof with the same house. If you're going to have one roof arched make them all arched. (its a theme remember) You're getting the colors to at least match. Your main problem right now is they appear dull. Try to add things that would give the buildings variety such as chimeny's on some etc..
Don't be afraid to expirement. The key to good buildings is simple. Don't look at the tiles for what they are, look at what they could be. Part of a lamp can make a bar stool. A staircase tile can make a floor, Heck even a bar table can make a roof.
You have to look beyond what the tiles where originally intended for. I would suggest perhaps trying to find a nice theme using tiles that are actually intended for something else. Some times if you get stuck its better to sketch the level in how you want it to look. Then examine the tile set and find what tiles would best match your sketch. That's what I do when expirementing often times.
