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When you finish the original game the first time, a new star designation goes beside your save account and the game starts over with some variations to make the game more difficult. For example, in star mode, all of the Goombas are replaced with Buzzy Beetles and all floating platforms are considerably shortened. The effects of this mode do not consistently increase the difficulty (unchanged enemies pose no more a threat than before) but completion does prove much more challenging for platforming-intensive stages. When you beat the game on Star Mode, the game lets you choose which level to play and on what mode in the account selection screen.

Super Mario Bros: The Lost Levels

Other than the main game, the Japanese version of Super Mario Bros. 2 (aka The Lost Levels) is in this release. The format of Lost Levels is nearly identical to the original Mario Bros mechanically and graphically but it is designed to be far more difficult than its predecessor (it's also much more difficult than Star Mode). In addition to demanding better timing and greater levels of finesse, the game actively tries to trick the player with bogus secrets and red herrings, the most notorious of which is a warp zone that takes the player back to World 1 from later Worlds. Lost Levels is also the relatively obscure debut of the Poison Mushroom which will harm Mario when touched.

Challenge Mode

One of the new modes is a challenge mode where each of the original 32 levels have special items hidden in them for Mario to find. The player can choose any level they have beaten on a save in the original game. Challenge Mode has three goals for each level: finding five special red coins, finding the Yoshi egg, and achieving a score by the end (determined by the development team). There is only one Challenge Mode account and it can be reset if desired.

You vs. Boo

Another new mode is a versus mode either against Luigi or Boo in a race to the flag. This can be done either as a singleplayer to two-player game with a link cable. The objective is to dodge a level full of obstacles, traps, and hazards to beat your opponent to the flag. There are eight levels to choose from and though they are graphically similar to the original game, none of the level designs are taken from the original. Each map has a series of buttons that players can hit to change the status of certain blocks, altering their existence or spikiness. There are two states the buttons toggle and one is beneficial to each contestant, striking a balance between making sure the layout is ideal and just gunning for the flag.

Boo is your opponent when playing against the computer and he simply floats by at a constant rate in the background instead of being blocked by obstacles, making these much more of a race against the clock than the multiplayer version. However, the computer will switch the state of the blocks in the level at a fairly constant rate, so this danger is not averted.

Toad's Toy Box

Every time the player makes significant progress in a mode or in the original game, things open up in Toad's Toy Box. They are just random throwaway items like a calendar, a fortune teller, and a songwriter for Peach that can affect the opening music for the game. One item worth a mention is the "Where's Yoshi?" section, a randomized hint machine that will choose random levels and show the approximate location of the Yoshi Egg in them for Challenge Mode.

Secrets

Start a game with 10 lives instead of 5

to start a game with 10 lives (new file), go to the toy box, and select the fortune teller option. now pick a card. keep on picking until you get an "Extremely Lucky" card, and you should see a 5-up on the card.
now you can start a game with 10 instead of 5 lives.