I think we may have found the killer appeal for the game boy!
I just have to talk more about the revolutionary gameplay! As you get further into the game, you will feel determined to solve all the puzzles (similar to zelda dungeons) and bump up your poke'mon to level 100 in order to ace the poke'mon league. Ok, I know this is not exactly gameplay, but there are so many glitches to exploit ( Just like te infamous Missingno. trick).
Next, especially for a GB, the graphics feel like a colourless NES game. For those who don't know yet, that's very good. The graphics are not quite as bland if played on a gameboy color or advanced because you can play in a single colour and change it by pessing buttons on the GAMEBOY screen with the two dings. For a GB, they are stand out better than many other GB games because the actual vision of the gameplay looks not as blurry as the otheres.
Finally, I like the sound because although the graphics are similar to a NES, the sound is better. This is because the NES usually has the bass part of the music louder than the melody. Therefore, you'll get pretty much just deep pitched beeps in the background. However, poke'mon's sound, just like The legend of zelda (NES) equally blends the bass, melody and harmony. To sum up, FANTASTIC game that any poke'mon fan must at least have a go at. Once again, WELL DONE,NINTENDO!