Some additional tweaking with curves.

MOST digital camera images benefit from judicious sharpening.
Select {Filter - unsharp mask} and play with it, see if you like the effect.

This is NOT a REAL unsharp mask, more of an edge enhancement. There is a VERY effective unsharp
mask technique for Photoshop which can have an enourmous effect on images - particularly nebulae -
which emulates the real unsharp mask method used with film and glass plates. Go to
http://www.astropix.com/HTML/J_DIGIT/USM.HTM for a good description. While you're at that site, I
recommend considering either or both of Jerry Lodriguss' books on DSLR Astrophotography.