It really took a lot of the best elements from Super Mario Bros. 2 like one-way background platforms, pick-up capabilities (now mostly limited to shells/bombs) and big Mario's new sprite. Then it combined it with some of the best elements from the original Super Mario Bros. like little Mario's sprite and breakable blocks. Finally it added a bunch of new elements of its own like slopes, and both horizontal and vertical scrolling. The end result was the most complete Mario platformer and a pretty remarkable technical achievement for the aging NES.