MEGA MAN ZERO / Capcom / Gameboy Advance
To understand Mega Man Zero, you need to first understand the direction of the Mega Man X franchise after its first installment; specifically, you need to understand the demographic that these games are squarely and exclusively marketed at.
They're very difficult run-n-gun/platformers whose level design takes a rather sadistic "learn by getting killed unexpectedly then doing it again" approach, coming to a head in frantic boss battles that require you to memorize complex patterns while keeping up an almost constant stream of precision motion. This is aimed squarely at pre-teens, who have the free time (and the inclination) to throw themselves through a punishing level or boss battle over and over and over and over and over and over ad nauseum until they perfect every little nuance of it and can do it all from muscle memory; pre-teens are also the only ones who can possibly stomach the corny Saturday cartoon melodrama and posturing that serve as the game's story.
The first Mega Man X was the most popular because it was the most broadly enjoyable, tuned and pitched to a wide audience. The second game took a sharp turn in the "hardcore" direction and the series would lean to various degrees in that direction for the rest of its life. Zero, however, comes out of the gate entirely pitched to the "hardcore tween" demographic, with no further concessions made to anyone else.
This isn't to say there aren't adults who enjoy the "do the same difficult shit over and over and over and over" platformer style, but it's polarizing - either people really go for it or they think it's a complete waste of their time and quit early. As an adult you'll also be skipping/enduring the corny characters and melodrama rather than getting involved with the game's world; what I really think will drive off the grown-ups for this one is all the extraneous shit they've belabored it with.
You equip "cyber elves" which can give you little perks and abilities, which is fine as a basic concept, but the execution adds a tedious grind element as they appear as random drops, have to be "powered up" by giving them energy crystals which also appear as random drops, then disappear once used, forcing you to go look for a new one. There's also a "hub world" base that you operate from, which is needlessly padded out with tons of empty space that you have to tediously walk through to get between basic game functions that the other games used to nicely condense for you in between-level menus.
And unlike in the mainline series and X, you don't select bosses to take on in order, but rather have to plow through a linear progression of interconnected levels. I guess the attempt was to introduce more of a Metroidvania element, but it falls flat.
* Gameplay Video