I mean Undertale is less a game and more a deconstruction of what we expect in games, so while the novelty of the story and characters might explain why tumblr latched onto it, its real strength is all about its ability to mislead players.
Basically Undertale is one of the few truly imaginative games in recent memory
Yes
The game focused on one emotion and nailed it 1000%: surprise
Fuck the haters who think it's too nerdy/lame to think of games deeply in this way
It's funny because someone above post about how they love the battle system and I'd argue it doesn't even really have a battle system.
Battle systems are usually based concepts like stats, strengths/weaknesses, various offensive attacks and yet the game's only got one type of attack (a physical one). Also, the fact that the attack itself is also wildly consistent. In the battle with Toriel the attack power is rather consistent... until you do a massive amount of damage and kill her. In the battle with Sans you literally do no damage and miss every time until the very end. I mean for standard random encounters I guess it applies, but only if your goal is to kill (or if you still don't understand so far into the game that you don't need to kill). And the bullet hell stuff is basically all defensive.
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TheRock ~ I had a name, my father called me Blues.