What is so good about it? I haven't played it, but from what I've seen it looked like a fairly generic monster shooter, except without a cover system.
But I kept playing, and started getting used to the movement and the shooting and started dancing through the enemies and dodging fireball after fireball while charging headfirst into a room, picking off enemies easily even though I'm using a controller (I know it has some auto-aim assistance but it's one of the best I've seen) and it felt different from every other fps I've played lately. Last game in a long time to make me feel that way was Splatoon.
The monster AI is better than in most games too. They chase you, leap onto ledges, climb up ledges and hang from walls, run around to get behind you (sometimes), constantly moving to dodge you and get a better shot at you, not hiding behind cover and then appearing briefly for you to shoot where you've been training your gun for 30 seconds like most games. Not the best I've seen, but better than most fps games.
Then there's the secrets. It's hilarious how Doom will show you the secret, either through a window or behind a vent, or a hidden level section will show up on the map, and then it pretty much dares you to figure out how to get there. You go where it looks like you can get there and there's nothing there. It can take a long time to figure out how to get to some of these secrets. That's pretty fun also.
The need to shoot people enough that they can be glory killed, and then needing to rush in to get that melee kill because it drops the health you desperately need, or hit them with a chainsaw to drop ammo you need is great too.
It's super smooth, and the levels are giant, and puzzly, it's pretty, and just a lot of fun. What Doom should be.
For me, it's a similar feeling to the original Doom, albeit in a very different (one could say "more mature") packaging. It's not trying to be a modern FPS as we know it, it's almost like a parallel universe version of a modern FPS. It's really masterly done, any way you put it, but you only realize it as you play through it. When watching a video, it's too easy to get distracted by the superficial qualities of it and miss the meta-game.
Plus the frame rate is amazing.
(In general it's how most games already work; it's just that it's normally convenient to set it at a fixed rate to make some things like physics more deterministic, and it gives a nice target performance/budget).
There aren't that many games like that any more. They focus on other things.