The game's writing is basically like Japan's visual novel genre where you're decisions affect the dialogue ever so slightly and effect which ending you get.This is really selling the ramifications of your decisions short. The game's events and tone change radically depending on how you approach it, and not in a "okay, which of these 3 menu options do you pick at this arbitrary juncture" sort of way. It's incredibly perceptive and well thought-out, and I can't imagine the kind of time and planning that went into keeping all of the connections straight -- especially since this is effectively a one-person project.
I've also seen the "it's like Earthbound" line a lot, with the implication that Undertale is a pale imitation of another game series, but I can't stress enough how much it's really doing its own thing here.
If you hate a lot of story and play RPGs for their gameplay this game is not for you.This is really interesting to me, since I thought Undertale had much better gameplay than any JRPG I've played... maybe ever? In terms of doing interesting things with its mechanics, subverting expectations, and keeping you constantly engaged, it's definitely up there with some of the best. If you consider "gameplay" to be number crunching subsystems, then you're going to be disappointed, but what's actually there is really, really strong.