You got me here... this is one of THE MOST beautiful nebulae pictures I have ever seen... I'll have to write the critique piece by piece.
Color: This may be a swayed perspective for me, because these are all my favorite colors in one huge awesome thing... the color itself glows from within, even though no huge amount of white is used. The 'strings of light' used inside are incredibly realistic, so well done that I feel that I know the structure itself. the black dust cloud surrounding the nebula has a sort of 'fuzzy' feel, and looks precisely like the photographs. You can also tell that this cloud is encasing the nebula, and get a pretty good feel for the distance and dimension.
Detail: The addition of the stars in the background is very good, not too many to make it look silly, and not too little to make it look like some bizarre plant instead of an outerspace scene. The milky, watery look inside of the cloud itself is gorgeous, giving it a 'light' feel. The tiny planet also gives you a good idea of how big it is, and wraps that 'area' together, as well as that fragment of cloud floating towards the middle.
One thing I might have changed is the amount of detail... in all truth, there might be a bit too much. This is hard to say because the picture is stupendous, but when I look at it, the veins of cloud look a bit too individual, there is not enough of a gradient buildup around them. Also at the bottom right corner, that sort of yellow line out of the more golden hued clouds... I'm not sure what is wrong with it, but it doesn't 'feel' right.
Still, in the face of all the other things, those are less than minor.
Alltogether this is excellent: The many different blues and greens give the nebula a special feel, since most other artists go with purples, oranges, 'warmer' colors in general.