Okay, so I've been staring at this photo for far too long, and I can't tell what's good anymore at all.  I'd love to hear some opinions and advice on how to make it look less cut out.  Thanks!

Actually - you need to cut out the "single" hairs to by zoom it in to pixel size and cut them out to. Maybe a pixel tollerance of 2 would be good too...

Another "trick" would be double the cut out layer - put some "hard light" on it and add extreme gaussian blur

But still - a good job so far - please allow me one question: Couldn't you find some other color for the background?

My advice to get rid of the "Cut-out" look would be to add the stray hairs from the top of her head to the photo with the dark background. Without them it looks like she has a cap on her head which is giving her that "Cut-out" look.
Not so dark of a background would be good but its entirely up to you and what your going for. The stray hairs is the only thing i see that really stands out, add those and it should look better.

Are you drawing paths or using the magic wand?

If the part of the image is not in sharp focus, don't make the transition sharp, or it will look like she is cut out and pasted in.

You can also play with layer modes (darken, lighten) and then erase the parts of the BKG that darken her face and highlights in her hair). The new BKG is darker than the old one but lighter than her hair.

You're welcome :-)

Some suggestions:

Don't cut all of it out, only the most offensive strays.

Use the clone stamp tool with Airbrush turned on, and sample the background layer frequently.

If you're going to change the background, make sure it's a color that works with the warm tones of the model's hair and face.

If you've been staring at it "far too long," put it away for a while, the take a fresh look in a week or so (assuming you have that luxury and it's not for a client who needs it RIGHT NOW.)