Zelda 2 is really flawed, but it

• Expanded NPC dialog and world-building that hints at the location of key items and upgrades or some semblance of "side quest" design (excluding old man in caves original zelda design, that kind of counts too granted)
• Has a decent combat system with high/low/block and interesting enemy design, arguably leading more diversity of monsters than the top down Zeldas.
• Featured a "leveling" system that gives players choice between magic or health depending on playstyle (this would eventually return with how upgrades worked in BotW or TotK, and was removed from the US release IIRC)
• Has some decent songs here or there.

Granted, all of this is ignoring that the games difficulty is crazy hard and basically requires you to play with a guide if you want any chance of playing the game well. However, the reaction to the 2D aspect is far overblown when you consider that the original Zelda also features 2D platforming segments that weren't as well designed as Zelda 2's. Zelda 1 is the better game, mind you, as the design is more punctual due to the lack of the systems above, but I think that OoT starting as effectively a remake of Zelda 2 says a lot that Miyamoto wasn't quite ready to give up the ideas from that game (and I think helped inform the basis of the z-targeting system, which simplified attacking and defending mechanics.)