okay, now that I've actually completed outertale pacifist (at lv 0), here are my official thoughts
Like I said before, there are a lot of distinctions to make this game more than Undertale in space. Obviously tileset changes, map layout, character designs, battles... I really like a lot of the attack designs. They're different enough you're like :0 but not so weird and difficult you can't get past them (except for gerson go fuck yourself gerson). I feel like some character dynamics were changed a lot, like Sans and Papyrus and Mettaton and Alphys. Sans and Papyrus's relationship changed kind of subtly, with it mostly being dialogue differences that gave it a different vibe. Mettaton and Alphys were a lot different here. I think the dev wanted Alphys to be a "better" person, so both the pretending-you're-in-danger-to-save-you and amalgamate plot lines were abandoned. The former is what changed the Mettaton-Alphys dynamic, obviously lol. Unless I missed something, it seems like Alphys and Mettaton weren't really great friends, and it's never addressed in a meaningful way, if at all. I'm also sad about the amalgamate thing being dropped, because it's a huge part of Alphys' character and the true pacifist route. It's a big example of how even if you messed up in the past, you are not just your mistakes and you can be a better person. I do kinda think that was probably dropped for runtime purposes, if not for the reason I stated earlier, which I'll get to later.
On the subject of character changes, I think that Toriel was changed a little bit at the end. In Undertale, she feels righteous and correct in her opinion that Asgore is a little bitch and chides him over it right before the Asriel battle. In this, she feels remorse for the way she treated/thought of him, and that's something she seems to grapple with for the rest of the game. I went to find Asriel before checking on her returning to the group, so I don't know how it resolved :( I'm not too sure that's a change I liked, either, but it's more fitting with how the story handled the fallen humans. Instead of killing them, Asgore asked them to give up their soul power and wait for the next human to come, which they all agreed to. That's a far cry from killing them all and waiting for the next. Okay on to things I totally loved
LOVED the neutral Flowey battle. The way all the different soul types were incorporated made it a challenging and fun battle. And then, for pacifist, you had to walk around to wake up the latent soul power. I think it's cool that in both Justice was the most "awake", per se. I think it's either because they were the most recent to arrive or because of their implant that fucked up the stasis chambers. In regards to runtime, the pacifist thing was VERY long winded and took up a lot of time. I think I personally would have preferred it to be a little shorter, but all of that replaces the True Lab, essentially. Another piece of Outertale making it's own. And it was cute! I liked seeing little glimpses into the humans' pasts.
So, final word... I liked it. Not everything, not every piece, but as a whole it was super good. The intent was to make it like playing Undertale for the first time again, and it definitely delivered.