Greed is my favorite, actually
I based it on the old myth that barnacles are baby geese, and my concept has it working a little like an oyster in that there's "treasure" inside the shell. Treasure so heavy, it can't lift its blind head off the ground.
I thought it would be perfect to make Lust a baby-like monster, since babies are a consequence of sex. Possibly the most horrific consequence possible, of course.
Wrath is meant to look helpless and malformed, which would be the reason it's so hateful of everything else. I'm honestly not a big fan of "aggressive" looking monsters. I find weird and sickly a lot creepier.
I've never though of envy as an "upset" state...it's more of a "longing" state. Since it comes from a dissatisfaction or lack of appreciation for what you have, the Envy bird has a weak, pathetic and minimal body - it doesn't have anything to be happy with.
It's a nice gesture, but I can't really use a critique on these guys, since their designs are absolutely final for projects already in the works. They've built up some fans, too, who would hate to see them change