The levels are short but the way they are made makes up for that easily because, unlike most Mario games, there are no save points in the levels. It is done so well that although some levels are very frustrating you never get the feeling that it's so difficult you can't finish the level. The levels are so cleverly constructed and challenging that you can't put it down easily.
So what makes it so special? Well, as I said, the gameplay is some of the best ever in video games. It combined the brilliant gameplay of part 1 with the graphical quality of part 2 into one massive adventure that became the best selling game of all time. When it first came out on the NES this game was amazing. The gameplay is probably the best thing about Super Mario Bros.
