The biggest mistakes I see people make are making insides levels too large, using tile incorrectly, or putting too little or too much detail. There are also just plain ugly levels.
Your first level is a good first try. Make the level smaller, and see how that looks.
Your second level isn't all that great. You should avoid dragging arrays of tiles like you did with the walls and floor.
I also noticed that you are using pre-made tile arrangements instead of going through the tileset, which isn't really bad; however, you aren't going to learn where tiles are if you stick with the pre-made stuff.