As a kid, I loved it because I thought Bunny Mario was awesome. Still do, I mean, why haven't we seen that powerup return? My favorite music in the game was actually the first stage (the training stage I call it), before you go to any zones. Oh, and then there's the cool maps to each zone, the cool map of the overworld, how the Mario Zone map would gradually reveal parts of itself as you went along... and I have a fondness for the Tree Zone boss. Really is a lost treasure of a game. Leagues better than SML 1. Still a hard game.
While I think SML3: Wario Land is a better game overall, SML2 just seems to -look- more appealing.
The genius of Wario Land 2 and 3 made me forget all about the previous games though.
And wouldn't you know it, it's the one Game Boy game I lost a long time ago.
Also: First game I experimented with when getting the Super Game Boy, the pig-cannon enemy from the Mario Zone is cute, and the covers for the three Super Mario Lands are all very cool.
EDIT: Just played through the entire game and beat it. Actually, this game was easy. Also loved that every stage had a different look and that most stages had a particular platforming element emphasized. I can't be the only person who tried recreating the final level of the Mario zone (the Lego level) with real Legos.