Much more polished than its predecessor, Super Mario Land 2: Six Golden Coins is a decent pocket Super Mario World.
The Bad: Physics are still a bit akward; ridiculous difficulty spike at the final level.
After a messy debut on the Game Boy this sequel manages not only to put the Mario series on the right track but also to develop a "Land" identity of some sort by not denying the first game entirely while bringing some novelty to the table.
Super Mario Land 2 is organized in the same fashion its console counterpart Super Mario World (SNES) was before it: levels scattered around a big map with smaller thematic "zones" (as they're called here) and their respective bosses. Each boss holds a special coin (the "Six Golden Coins" of the subtitle) which are required to break a magic seal locking the front door of Mario's castle... where a brand new archenemy (Mario's antithesis Wario) is hiding.
What's interesting here is how old features and new concepts were mixed together to a great result when it comes to a cohesive overall design. The simple "hard exit = bonus level" feature of the first SML makes more sense now since you may really open an alternate route in the map when you find one (aside that you must ring a bell to do so and the same goes for the checkpoints in the middle of the stages, making it a less random item).
Problems with the color pallete (or the lack of one) were solved with the creation of entirely new costumes (and its hats) for Mario's power-ups. Both the old Fire Mario and the new Bunny Mario costumes twist old moves well to a particular gameplay balance emulating SMB3's racoon tail (no flying here, sorry) and SMW's spin jump. Not to mention the cool space levels, once again quoting the first SML but exchanging the nonsensical spaceship for an astronaut suit--which pretty much nails it definitely.
On the down side the level design is a bit too bland and some gimmicks (like the "jelly" that slows Mario down) are overly used, but those would have been small complaints... if it wasn't for the final challenge.
Mario's castle sends the dificulty level through the roof all of a sudden and its length, lack of checkpoints and multiple bosses seriously depose against the final experience.
In the end Super Mario Land 2: Six Golden Coins is a gust of fresh air to the series and at the same time it settles the standard for portable Mario games.