Definitely these two. Squash and stretch is better than nothing, but skew changes everything. It's seriously the difference between making rigid XY axis animations and making animations that appear to cross the Z axis. As for placeholders, that would be amazing to be able to draw a placeholder, take it back into Photoshop, and know the exact size and shape to work on.
Beyond that, stability is big for me. I mentioned it in another thread, but Spriter is pretty much unusable for me right now. I'm patiently waiting for a build that works, then I'm waiting for a way to get it to play nice with Unity without having to spend even more money.