I'll put it out up front: I like what I'm seeing in Shadows. I'm looking forward to playing with these cards and I think this set has got a lot going for it.
I still don't like double faced cards: I'm never going to like 'em, let's just get that out of the way now. We're agreed to disagree and I'm good with it.
Delirium: this is a neat idea that I wonder if it will work well. The obvious comparison for me is to Tarmogoyf (not the Threshold mechanic), but Tarmogoyf looks at everyone's graveyard for its bonuses. That's a pretty significant drawback and once again, I believe that this mechanic will be a lower tier Limited one. This is both too bad, because the mechanic is a neat way to provide a bonus, but good since it means that the price of fetchlands will remain consistent.
Although if they push the mechanic in Eldritch Moon, expect this to all get turned around and I personally have a R/G Tarmogoyf deck that will love Delirium if there are cards worth playing.
Investigate: it looks like they added 1 to the cost of most of these cards and then let you get a Clue token which can be sac'd for 2 more. So the mechanic has a additional cost of 3, which you can pay over time, to draw a card, instead of being a straight cantrip. From a flavor perspective, it makes sense. Mechanically, it's incredibly boring at the moment. If there's a sacrifice theme in the next set or block, that might become more interesting, and pairing these Clues with things like Arcbound Ravager or Daretti, Scrap Savant. Otherwise, yay Limited stuff.
Of note: they don't spread the card draw very well and I think that's a mistake. White gets Investigate, which is good, but Blue also gets this and it's unnecessary. Red and Black could both benefit from this mechanic. Card draw is too powerful to provide unevenly.
Madness: here is where things get interesting. There are plenty of cards that use discarding as a cost so you can save your mana for the Madness stuff. The cards in SOI don't seem broken or crazy but they're definitely cool and I'm glad this mechanic has come back. More interesting stuff like this WotC.
Again: this ability is really powerful and I'm sorry to see it spread unevenly. Hopefully Eldritch Moon will give Green and White some Madness cards.
Skulk isn't a mechanic but I do like the implementation and the feel of it. I wouldn't be surprised to see this become an evergreen ability.
While it's cool to use the double-faced mechanic to show Avacyn's transformation, I also feel that this displays a certain lack of imagination on WotC's part, unable to give us an example of a mono-white villain.
Damnit, zombies are going to make my life miserable again, courtesy Fuz.
Why wasn't blue made the color of player mill and black opponent mill? It makes more sense that way: Black will peel back the opponent's mind, Blue will delve into its own.
Werewolves are still the worst tribe, and Wizard's attempt to allow players to control or make this tribe desirable result in making it feel more pathetic rather than less.
I know Westvale Abbey has everyone by the nuts right now but I don't think that will last. The payoff isn't worth it, at least not currently. If W/B lifegain control or tokens can make a dent, then maybe. But putting all of your eggs in one basket is rarely a good idea and the delay to get to 5 creatures and 6 mana is very, very risky, even if the payoff could work.
Also, from a nerd perspective: how does a chapel become a demon? I get having a sacrifice that creates a demon but buildings don't become creatures. And if I'm going to nitpick (and I am) then shouldn't Westvale Abbey be a legendary land, since it provides a legendary creature? How many Westvale Abbeys are there?
I think it's super awesome Scarecrows are coming back as a creature type! But I also want to go back to Shadowmoor so....
There's been word that Cryptolith Rite is just the most broken thing ever. Nope. But it is quite good.
As much as I dislike double-faced cards, I am glad to see them trying some new things, as with Skin Invasion or Startled Awake.