Beat Tatanga, get Daisy, and make sure Toadstool doesn't find out.
For starters, you have to give credit to Nintendo for not trying to just port a dumbed down version of NES' Super Mario Bros, instead they made a game with similar mechanics (and some new ones, like the shooter levels), but a completely new quest, one aptly named "Land" ("World" was too big for a GB game, specially one as short as this one).
The graphics obviously can't compare to those on Super Mario Bros, which is not shocking, considering the developers were still years away from tapping the GB's real power. Some things look taken out straight from a Game & Watch, some others, like the bosses, look pretty decent.
The music a different story, even though it can get a little repetitive, what little there is, it's absolutely beautiful. Cheerful and fast, you'll find yourself humming some of the tunes during days.
The game IS on the short side, though, with only 4 worlds with 3 levels each, you'll reach the end in no time (specially because the last levels don't give you time to goof off, you'll have to run through them). The boss battles are cool, but they're not particularly challenging either, pretty much like the rest of the game. As long as you make sure you get to the bonus stage at the end of each level, and get at least one extra life, you should make it to the end with a lot of lifes to waste.
All in all, it's a fun game, with great music, average graphics, and decent challenge, that I still play to this day, even if it's just a couple of levels.