The lowdown:
Considering the last Super Mario Bros. game is still the best selling video game of all time (17.28 million copies and counting) this has a lot to live up to, but it almost effortlessly captures the action and charm of the old games. The basics of the game are the same as always as you run and jump through mostly horizontally scrolling levels. New to the game are power-ups that turn Mario into a giant or make him really tiny, as well as one that gives him a Koopa shell on his back to attack enemies with. Although the gameplay is all in two dimensions the graphics are 3D, with all new moves such as the triple jump and wall jump added in as well.

Most exciting moment:
As well as the single player mode there's also a wireless multiplayer mode where you get to battle a friend (as Luigi) across a whole bunch of specially designed levels.

Since you ask:
As Super Mario World on the SNES (since re-released as Super Mario Advance 2 on the GBA) was subtitled Super Mario Bros. 4 in Japan that name couldn't be used for this new game, although that's secretly whatever everyone's calling it - behind Nintendo's back.