These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The overworld music is comparable to the Bionic Commando main theme. Due to the high encounter rate, though, you won't hear much of it unless you stand around.
The song used in Westside City and outside Neo City.
The Bronze Rocks military march.
Demonic Spiders: B. Lance has an erratic bouncing pattern, a capsule that lets them go twice, a fairly high encounter rate, and they're powerful even if you're at a good level for other enemies in the area.
In later dungeons, B. Dragons and B. Zeppelins pick up the slack.
I Am Not Shazam: The robot that follows you and NPCs around is a Metal Walker. Enemy robots and those controlled by Metal Masters are Metal Busters. There's a difference.
Tear Jerker: When you reach Ever Green, you start looking for the professor the enemy Metal Master told you about. He's dead; you find his grave. Then a woman approaches, thinking you're a bad guy, and attacks. She's his granddaughter. She was fighting for his memory.
B. Dragon. He does a fair amount of damage even if you grinded... and his amount of HP is massive. Even if you do 77 damage with each hit, he will likely kill you before you kill him, especially if he or his minions get good capsules.