You won't mistake it for a console game, but Super Mario Land 2 is nevertheless a huge step up from the prequel. The sprite work is somewhere between Mario 3 and Mario World, the gameplay is much improved and overall it just feels so much closer to one of the console entries (just a bit of gameplay slop and jank aside).
Surprising, then, that not only did Shigeru Miyamoto not direct this one, it was passed off to new director Hiroji Kiyotake. He had been Gunpei Yokoi's right-hand man on his big NES hits, and he would go on to helm the rest of this series as well as much of the Wario series on later systems. In this one you can already see some of his imprint on the series - a tendency to slower-paced and more non-linear / exploration-focused levels, minimalist music and a wider variety of more challenging bosses than the usual samey assembly of Koopa Kidz.
Of course, it's also the origin of Wario (created by Kiyotake), here in the role of ultimate nemesis. Mario has his own castle apparently, but Wario took it over, and locked him out with some sort of special gate that requires the titular Six Golden Coins to open. Which, of course, are strewn about the land in a variety of imaginative and well-designed (if usually kinda short and easy) levels.
New to this one is a world map, which shuttles you between areas and has a few optional pit stops (like a slot machine you can pump your coins into for extra life bundurus). And though it isn't as exploration-rich as Mario World, there are at least seven levels in the game that have secret exits to a bonus level. The map here is also totally open from the beginning, unlike Mario World - you can tackle the six sub-worlds in any order you care to.
Mario Land 2 is also the origin of Mario's "screw attack" jump that breaks bricks he lands on, though a quality unique to this game is that it also triggers the "?" blocks when you hit them from above. The familiar fire flower returns, and the fireballs actually have the proper physics this time, but it's paired with the new Carrot, which gives Mario a set of rabbit ears that allow him to parachute gently back to earth after jumping. The carrot wouldn't be seen again in the series, but the mechanic of tapping the button to hover down would be re-used for other power-ups in the more modern games.
As mentioned, the game has its sloppy and iffy moments in the gameplay department - collision detection rules in the boss battles stand out, and Mario in general just has a bit of a slower and stiffer feel than in the NES / SNES games. His little signature moves that were missing from the prequel have been restored here, however - sliding a bit after landing from a jumping run, the ability to freely wongle him around in mid-air, and so on. And since every level is accessible from the get-go, there's really no difficulty curve - every level is very much on the easy side, though I wouldn't go so far as to say the game is a total cakewalk. Experts at the 2D console games will likely find everything up to the final level trifling, though.
The creativity in level design and art really does a lot to make up for any shortcomings in gameplay, however. As with the prequel, this feels more experimental than a mainline Mario title, with a lot more wild level types and enemies. The overworld map and sub-map art is fantastic, and levels have a lot of little flourishes to enjoy. As mentioned previously, music does tend to be more minimalist (done by K.K. Slider here instead of series mainstay Koji Kondo), but there are also some memorable tunes - I swear some fairly recent pop song totally stole the melody of Space Zone 2
... or maybe it was stolen from some old Miami Sound Machine song ... I dunno. Can't quite put my finger on it but it sounds a lot like SOMETHING I've heard on a radio at some point.
Though it's on the short and easy side, and feels floatier than the console games, Mario Land 2 is nevertheless one of the gems of the original Gameboy library (pushing the hardware to its limits with a 4 MB cartridge). The platforming still largely stands up today and it's definitely worth checking out.