To alleviate any concerns about the AI or difficulty, from what I gather by talking to people who went to the April 1st co-op events...
The AI is much more alert/elite (possibly linked to the low tier vs high tier enemies I was talking about), atleast on Realistic and Co-op. Apparently, if the enemy sees you long enough to prepare to take you out (just beyond the stage of surprise), then you'll have issues doing a takedown on them, and might get countered. Possibly only in co-op, but it's neat.
Also, when you get countered/knocked down by said enemies (or downed by low health), the enemy can take one of you hostage (you can smack him, but if you put up too much of a fight, he'll execute you).
Apparently Realistic and Normal/Easy are severely differentiated in the full game. Don't think there are any HUD differences, but plenty of gameplay differences.
I also heard that there isn't always a shmuck in a dark corner waiting for you to take him down, so you can M&E everyone else. This means you'll have to consider your options.
Neat, just solidifies my purchase a little more
Again, if you're getting SCC, add me! (GT: Rhett819)