There are plenty of gameplay related reasons Other M was horrible, we don't even need to touch that horrendous plot. Here are 10 gameplay ones:
1. The control scheme is awful, switching between missiles and walking makes you feel less powerful not more.
2. Name me a new power-up that this game brought to the series. Other M rehashes old weapons and brought nothing new to Samus' arsenal. She looks just about the same at the start of the game as the end.
3. Health regen is a bad system and recharging is a broken mechanic, doesn't add to the flow of the game
4. Pixel hunting is just obnoxious.
5. The sector system is a Metroid Fusion rehash, and nothing interesting is done with it.
6. The game doesn't even let you know you've unlocked a vital powerup late in the game.
7. The game is painfully linear
8. Dodging is really powerful and gives you such a boost. Basically required because of the HP system.
9. Powerups are placed just out in the open for no reason
10. The game has a bad design for where you cannot access. I am surprised nobody has posted the jumping to the powerup gif yet but that communicates so much about why this game is messed up.
Now for plot:
1. Samus gets PTSD despite never having it in the past games.
2. Because this is her 'first time working with GF' since leaving apparently Metroid Prime series isn't canon (although it should be other way around).
3. It tries really hard to be relevant to both Super Metroid and Fusion, while making it painfully obvious nobody has any idea where to go with a sequel to Fusion.
4. The baby the baby the baby.
5. MB is just a really bad idea, also confusing.
6. The Deleter plot isn't resolved well
7. The post credits stuff is basically more Super Metroid tribute for no real reason.
8. Nobody cared about Adam enough for this game, nor Samus' history with him.
9. Plot yammering really broke the flow of the game.
10. The permission system just makes the player feel weaker, make up some excuse for why the suit is damaged or something don't make Adam more powerful for no real reason.