I'd say, add shadows. To all parts that should be dark, from hands, to chest area, but most importantly to the tail - it lacks even a light shadow. Probably a few highlights on some of the scales as well. The bracer is very undetailed, as is the blade, so add some detail to those (probably just use some textures).
But the main problem, and of course you did a great job and I can't expect you to really fix it, is the face. I mean, it looks kinda weird and kind of flat. The eyes seem too wide open, the nose and lips kinda weird. You see, it's mainly an issue of knowing where the texture will get extra stretched and ruin what you had drawn. Simple solutions I can think of now are remove that extra smile by lowering the corners of the mouth (she can't always smile, it's kinda weird.. like the Joker), round (as in degrees) the eyes a bit, add more shadows to the left and right parts of the face when the skin goes backwards (after the skull bones area.. you know what I'm talking about) and kind of shorten the nose's end part (or widen it, this could work too, depending on whether you want a really small nose or a more realistic one) because the texture gets stretched a lot. Face has to be the most perfected part of a texture, that is one of the reasons why I usually don't touch human faces a lot when retexturing (my skill level will never be enough to match other rendering stuff). Also, she looks kinda pale. I think you could try saturating her a bit, see if it works, if not, well, worth a try right?
Nevertheless great texture! I love how you rendered the metal parts with their highlights, trims and colors. The magenta to turquoise green is amazing, the cloth area has a good design.
I love the colors, this skin could be used by wow for any naga siren. 
