The game play of your level defines how the player plays through it, such as what objectives they have, how they get there, and what is the 'catch', how is the level they are playing through right now different than the one they just beat?
In the original SMW, nearly every level had a new theme or some type of special gimmick. This was easier fro the original SMW creators- they designed all of their sprites from scratch, so each level could have a new sprite, and be based off of the use of that sprite.
Creating a theme for a level can be challenging until you get a good idea. First, decide what type of level you are making- a Grassland, a Cave Level, or a castle, for instance, are all good bases. You can even come up with a hybrid theme, such as an underwater castle, or a water level that also has parts to play on land.
Once a base is established, a Hacker should think, what makes this level different from all of the others? The answer is completely up to the hacker. Different themes can come in the sprites used, the combination in which the sprites are used together, and how the land is based. A couple of examples:
Perhaps you need a lake level. What makes it challenging and different from the other levels? It could be made to auto scroll at a slow pace, making the player have to react more quickly to the obstacles. You could build the ground of the level so that the player will have to swim up to the top of the screen and then down to the bottom of the screen so Mario does not get squished. Then to make that a challenge, mix up the level with simple sprites such as fish swimming back and forth, then perhaps more blocks to limit the player ot a more restricted path.
Another example could be in designing a generic grassland. Make it interesting by combining sprites! Use falling bob-ombs to make the player watch out for whats coimng from above, and use blue koopas without shells to kick shells at the player. This presents a challenge to the player that might be more unique than previous levels.
Next to design, the level difficulty is very key. A player will not be interested in a hack that they breeze right through, but will become frustrated and quit a hack that makes it near impossible to move even forward by a couple of screens.
Difficulty for every level should be in a medium setting, placed on a scale of how far it is along in the hack. A level should be possible, but present a challenge that is fair as long as the player has a reasonable amount of skill, and room for error. Having room for error is where some hacks begin to crumble in difficulty. For example, say you have to make two jumps to get to beat a level, and if you miss, you die instantly by falling off of the platform and into some lava. This is fair, because a player has a reasonable challenge. However, if you place black piranha plants around these platforms, and give the player almost no leeway to make mistakes, it becomes unfair.
99 percent of the time, the players are human, and should be allowed an occasional mistake. Add power ups after very difficult areas, or in the middle of them. This will allow the player to get hit and recover, but if they get hit too easily,they will die before ever reaching that power up.
Finally, difficulty should be based on where the level is in the hack. Just because a level is in the beginning, however, does not mean a player should just breeze through it with their eyes closed, and the same goes for a level at the end of a hack, that requires the player to be perfect. The beginning levels of a hack should ease the player into the game, with only a few enemies that are easy to avoid, and simple ground structures. As the levels progress, make the jumps harder to do, ad in more enemies, include less safe spots with no enemies, and give the player a little less leeway as the levels get difficult.
The aesthetics of a level refers to the music, the graphics, and the colors used in a level. The number one most important aspect of the aesthetics of a level is that everything both fits the theme of the level and does not contradict with the other aspects of the level. If a background is realistic, then the foreground should also appear realistic. If the music is bright and cheery, then the level's graphics should not be dark and depressing.
The graphics in a level are how the ground and the background and the sprites appear to the player. Next to the actual graphics themselves are the colors that they use, or their palettes. Colors should not clash, in general, and extremely bright colors should not be used in excess, as it may hurt the player's eyes. The colors, as well as the graphics, should fit the level theme.
Many people will use custom music in their hacks. This is usually a good idea, as custom music can be made and used to fit a level theme more precisely. Again, the music should fit the theme of the level and match the graphics. Also, take into consideration how long a music piece is, and how long the level is. A short, repetitive little ditty should not be used throughout a very long castle level, and a long music piece can not be fully enjoyed in a short level.
A cut-off tile is a bit of land, or a block, or any foreground, that matches with another tile. when not matched with another tile, it will look awkward graphically and might distract the player from the level.
Generally, cut off tiles are not a large problem in the aesthetics of a hack, but if cut-off tiles are seen repeatedly, or in a very major part of the level, the hack will appear odd and be less appealing.