While making levels in NSMBW, it is important to be familiar with tilesets. Making errors with tiles will make your levels look less professional. Consult this list to see some common tiling errors that can be easily fixed.
In this image, Zementblock shows how tilesets usually work. This is very important information for newcomers wanting to fix their tiling errors. Of course, custom-made tilesets sometimes vary from this basic structure, but it is still helpful to know.
Using the corner tiles is very important so that the levels don't look unfinished and generally ugly.
Here is another corner error, this time on the insides of the corners.
Though it may seem obvious, some people don't know how to make multiple decorations on top of each other. Simply using different layers will fix this problem. Be wary of doing even the correct version of this too much though, as some people don't like too many background decorations piled on top of each other.
Very small holes in the ground simply look odd. Holes should be wide enough to be jumps, not too thin.
When a pow-block is used, the screen shakes, so all tiles must be extended one tile outside of the zone border to ensure no graphical problems. When pits are added to a level, Mario dies only after he falls beyond the zone, thus, pits should have four-tile extensions.
The top of the screen extends about 10 tiles above the zone, making above zone secrets like in original nsmbw's 1-3 possible. This isn't a tiling error, per say, but it is an important piece of information. The blocks at the top are placed so that players with the propeller mushroom cannot somehow get out of bounds.
When adding layer 2 solid-on-top tiles next to slopes, there is a possibility of having the player glitch through the floor with the small gap. Adding invisible tiles will sometimes help.
While this isn't specifically a tile error, it is good practice to make blocks three squares above the ground, not two.
Coins should definitely be placed in levels, because levels with few coins are quite bland. However, coins shouldn't be overly used in single locations. Coins should be thoroughly dispersed through the level, perhaps in jump arcs. If jump arcs aren't something you like, putting them on the ground is still possible, just don't let them be directly on the edge of the platform.
Here's another styling tip for coins on top of blocks.
Sometimes, people like to decorate their levels with rocks jutting out of the sides of the ground. This is a pleasing decoration but only when done right. The background should be filled in to prevent a hole into the BG of the level. Rocky dirt background tiles can get rid of this hole.
Another error with rock blocks is when they are too big. Make sure to use smaller blocks when decorating. Though it may seem more time consuming, it will result in a better looking decoration. Also, blocks of the same type shouldn't be placed directly next to each other if it can be helped.
This image is a bit confusing. The top left is problematic because when Mario exits the pipe, his feet can be seen at the bottom of the pipe. To fix this problem, the pipe can be extended. Unfortunately, there is still a problem. Both of the top pictures show a pipe going into solid on top platforms, which makes no sense. The two bottom pictures show ways to remove this solid on top platform.
This image of pipes had two problems. The yellow pipe should be fully behind the ground and the green pipe shouldn't jaggedly cut off the tiles of the ground. The Hole that the green pipe makes should also be partially filled with background rock as well. Another problem to some people is that they don't like seeing these two ways to make pipes go into the ground right next to each other, so choose only one option per level.