I honestly love the soundtrack to this game. I think it might be nostalgia, but I just loved quirky old GB music. Parts of it remind me of Link's Awakening, mainly any underground/cave sections. There's something about those little twinkly sounds that I love.
Overall, I'm actually surprised at how well this game holds up. It's slow, yeah, but I love what they do with the levels and how they change. I wish more games with World Maps like this one were so dynamic. Mario World did it a little bit by letting you find bridges and stuff (and then the level would be a bridge, kind of like in SMB3 but more organic), and DKCR did a great job of it as well. I just love how, for example, one level might hold no secrets, but if you go back later in the game, the tide has come in on the world map, and the level is now flooded with water.
Nice touches like that do a GREAT deal to make the game world feel alive. I actually think it's something that Nintendo is sorely lacking with with their most recent Mario games. It's like they can't make a platformer anymore unless it has either NO world map, or a very limited/generic one.