I have 2 major complaints about this game, I'm sure a lot of people don't care as much as I do for some of the points I'm going to make, but they surely affected my experience, and some of my friends' as well.
1. The game expects you to care about all the characters, and fails doing so.Most of the main characters in Undertale are just too "in your face". It feels like Toby Fox tried too hard to make you understand and care for his characters, and I think that's somewhat logical if we remember that the game's most important mechaninc is "sparing" enemies. If the player doesn't like a character, they won't make the effort of finding the correct actions to spare them, so the concept of the game falls apart.
Of course you can play without saving anyone, and the Genocide route encourages that, but I don't think any player will just save every enemy their first time, so the Pacifist route is impossible to access, and also the Genocide route, but I'll talk about it later.
At the beggining of the game, you find Toriel, one of the main characters. She stops Flowey when it tries to attack you and then guides you through the first corridor in the game like she's your mom. For me, this interaction was just a simple tutorial for walking and pressing buttons, so when she tries to force you into staying at her house and not letting you progress through the game, I only found it kind of annoying in the context of the plot.
Of course she's trying to protect you, but by then you have only played for like 15 minutes and I think it's impossible to get emotionally attached so quickly to a character you just met. She only helped you out through the tutorial, what do you expect me to feel? "I haven't even started the game", is what I exactly thought.
So when Toriel "fights" you because you want to scape and she doesn't let you, I had to kill her. It was the first "bossfight" and at this point I don't think any new player is really confortable with the combat system, so fighting her seems like the logical thing to do.
And this happens with some of the other main characters: Papyrus was just dumb to me, some of the stuff he says is kinda funny, but I never felt like wanting to spare him.
The same goes for Alphys for example, who in my opinion is the most annoying character in any game I ever played
(Even though you don't have to fight her)There are some characters that I like
: Asgore being my favourite, Sans, Asriel, and maybe Undyne (only when she's your enemy), so it's not like I deeply hate the Undertale characters.
The point is, if you don't even care about the main characters of the game, how am I supposed to spare every regular enemy or miniboss my first time playing?
The game gives you items like any other RPG, like weapons and food to recover life, so the "sparing" mechanic never feels "so unique" to a point that you want to start doing a pacifist route the first time around. at least at the beggining of the game.
Which leads me to my second complaint:
2. The game expects you to play the game again, twice.If you made the "mistake" of killing any enemies your first run, you will just get the Neutral ending, which means that you HAVE to replay the game a second time if you want any of the other 2 routes, which significantly improve some bosses, characters, and lore.
The Genocide Undyne fight is pretty okay,
also Sans who is the hardest boss in the game, and Asriel, the final boss in the Pacifist route. Including the music that goes with them.
If you didn't enjoy the Neutral route your first time playing, is almost imposible that you will try and play the whole game again. It's possible to get to the Pacifist route after the Neutral one without restarting your game, but you have to spare everyone your first time, so as I said earlier, that's just super rare. Also, you won't even know about the different routes if nobody tells you.
So if you truly enjoyed your first playthrough, and you want to 100% complete the game, you will have to play it again, twice, with the Pacifist route being 2 times longer in playtime that the Neutral route, and the Genocide route being so tedious that it's just boring.
I won't go in detail about what to do in each route, so I'll leave here the Undertale Wiki page for both routes:
Pacifist RouteGenocide RouteIf you don't do that, you are missing out on the 70% of the game playtime and lore, even though you have probably seen some (or most) of the events in the Pacifist route if you started sparing some of the bosses.
To wrap everything up, I think this game can be good at times, the battles are fun, even though they can be too simple when you are not fighting a boss, the music is great overall, some of the characters are well designed and there are some interesting and fun events and places.
Also the lore can be pretty interesting for what it is (if you ever complete every route)
Describing that last paragraph as "the good", "the bad" would be the game trying to hard to get you attatched to the characters, the tedious and boring requeriments for each route and the simple puzzles and gameplay in general outside the battles.
The "freedom" this games offers is actually very little, you can decide who you want to kill or save, but in reality if you really want to enjoy the game as a whole, you need to follow every rule and watch the same events over and over again.
at least in my relatively short experience and small knowledge with video games, most of the ones that choose to tackle violence just go "violence is bad because if you are violent then you get the bad ending, you don't want the bad ending do you?" and it kinda blows imo.
it has a relatively simple story that's not supposed to be all that deep and morally grey, simplicity is part of the charm here