I'm only just beginning the third level, which is probably 5 hours in or so?
I liked the original more, but I think some of this is just what I remember and what I don't. Keep in mind, I adored the original, so being below that bar doesn't mean I'm not really enjoying it. That said:
1) The original had better locations (thus far.) In the original, you start on the Mars base, which gets more and more corrupted, and then you end up in Hell. Here, you're in essentially the Hell levels to start. In the original, you were in places that made some kind of sense. You recognized office corridors, and spatially, it all fit. Then Hell was a pretty big shift. Here, you begin in things that are foreign, don't logically fit layout-wise, and are hard to visualize spatially in your head. To me, that's a downgrade.
2) Plot, too. The original was a story of a greedy corporation doing something bad and people making bad choices around it. With a layer of humanity mixed in to both why the corporation was messing and why bad decisions were made. Now, we're getting something more operatic with the fight between some race of something and Hell. But it's on Earth. I don't give a shit who the King of the Sentinals is, or who the Betrayer is. I just want to be some unknown being that is basically insanely good at killing demons now being sent to Earth to kill demons. I don't want to know where I came from. I don't want to see anything other than the remains of Earth's fight for itself, and demons to kill. I don't need much beyond this, and fleshing it out is making it worse, not better.
3) Humor. I feel like, in the original, it was played straight, with the deaths being funny, and maybe the secrets. Here, it's played in a few more areas, and there's a ridiculous hub that is loaded with humorous touches. They're throwbacks, sure, but they feel a bit out of place, and having to run around some giant hub is stupid. I don't want to walk down 4 floors and go 150 meters horizontally. I just want to click "kill more demons in next mission" or "kill more demons in a prevoiusly killed mission."
4) I'm cool with extra lives, but wish we could pick whether to use it or not. Having an extra life be used when I did something dumb and fell in lava isn't enjoyable, when I know there's a battle around the corner I'd much rather have it for.
All this said, I'm really enjoying the combat. I do think we should be able to hold more ammo, but I guess I can redo levels with cheat codes for real killing. Right now, I'm on Ultra-Hardcore or whathaveyou, the one below Nightmare, and finding it extremely fair. Not dying much.