This game is harder than the original Super Mario Bros., with fewer warp zones and power-ups, and more enemies. Six new levels were created for the game, all of which were reused in Super Mario Bros.: The Lost Levels.
Detailed set of level differences
The original version of Super Mario Bros. has some levels repeat, with an easier version with some obstacles removed (fire bars in the castle, fewer enemies, etc) earlier on, and then a later “full” version, e.g. 1-3 is the easier version of 5-3; 1-4 is the easier version of 6-4; 2-2/2-3 are the easier version(s) of 7-2/7-3; and 2-4 is the easier version of 5-4.
In the arcade version, most of the “easier versions” (all except 1-3) are replaced by the “harder version” to make way for new levels.
Some of the bricks above the end pipe in World 1-2 are removed to prevent anyone from performing the Minus World trick, although if the player goes to here, it is possible to see that the Minus World seems to still be in place.
The ground before the staircase at the end of World 1-3 is removed, resulting in the player either using a lift or performing a long jump to reach the staircase.
1-4 is replaced by 1-4 of The Lost Levels.
The Koopa Troopa at the ending staircase of 3-1 is replaced with a Goomba, making the infinite 1-Up exploit in that level impossible.
3-2 is replaced by 2-2 of The Lost Levels, which is similar, but with more obstacles such as open gaps and a floating pipe towards the end that the player has to use hidden blocks to reach in order to make the jump across a wide gap.
5-3 is replaced by 6-3 from the original game, and 6-3 is replaced by 4-3 from The Lost Levels. This has extremely long jumps to the platforms, and in one case, the player has to hop onto a Paratroopa next to the bluff (when it’s at the right height) to reach the platform.
Since 6 is a night world in both the NES and the arcade version, yet 4 is a day world in all three games (including The Lost Levels), the level is converted from a grassy daytime scene to a white-grounded night scene.
2-4 is replaced by 6-4 from the original game, and 6-4 is replaced by 5-4 from The Lost Levels, which also has a lot of long, hard jumps (and some tricky small ones), and an extra long firebar (which do not appear in the original NES version).
7-2/7-3 move up to replace 2-2/2-3, and have their slots filled by 6-2/6-3 of The Lost Levels. These are harder and longer versions of 7-2/7-3, with 7-2 (a water world) having very little floor, with stretches of low hanging reef, and 7-3 being an extra long bridge (with lots of gaps) with many more jumping Cheep-Cheeps (and flying Koopas as well).
The solution to the “maze” in 7-4 is slightly different.
4-4 and 5-4 trade places in the arcade version.
The invisible block used to reach the floating pipe in World 8-4 is moved one block higher, requiring a long jump to reach it. Also, the area where Mario/Luigi fights Bowser now has bricks above, making the area between it and the bridge narrower.