A few more comments now that I've finished playing the demo. I didn't really enjoy the new monsters--they didn't feel Doomy. If you wanted to you could use some other bestiary monsters, but unlike, say, KDIZD, I think Doom 2 already has enough variety in monsters that that's not necessary. Maybe for special boss fights.
I was also lukewarm about some of the changes to existing monsters. The triple-projectile Cacodemons were something I could deal with, but I didn't really like the Lost Souls or Spectres being on crack.
Map35 and 36, while beautiful, felt really out of place. I know they are meant as super secret type maps and aren't based on existing Doom 2 maps at all, but still, it felt like the mapping had gone away from just sprucing up the Doom 2 style. One of the things about Doom 2 is how abstract and varied its "hell" was and the sort of black and red gothic cathedral look feels like a whole different mapset.
Finally, a word about the advancedarmor powerup. It's neat to have, however, I noticed that when I picked it up, I didn't get the screen flashing red feedback when I took a hit. I assume this is because the armor was taking all of the damage and not touching my health, however, that's almost a drawback because without such an obvious cue I would often take multiple hits from an unseen enemy before I got clued in. I was wondering if there's a workaround that would let the screen still flash red.
Okay, so that's all the negative criticism. Now for what I loved about this, which is most of it. The mapping is top notch, and the intermission screens are cool without being annoying. I like that like KDIZD you're having the overt demonic corruption seep into the base levels right from the get-go. I can't wait to see what you do with the city levels, and with the Hell levels that are actually based off the id Hell levels.
On that note--I really hope this project is still moving along and doesn't die. I saw that there were several months between the last two updates, which is kind of worrying! Please keep posting progress reports.