As Jello has already pointed out, the lighting could be improved. When you start out in lighting, look at the eye sockets and nose shadow - get those right and the rest should fall into place.
The whole image seems a little desaturated. I'm guessing it is film and that maybe you've scanned a print? Hang on, you've got D50 in your sig... hmm. Looks a bit washed out anyway, perhaps have a play in PS, unless you like it like that.
The black of her trousers has lost its detail due to the contrast and lighting.
I'm not too sure about the pose, looks a bit like she's going to the loo! This is because it looks like she's shoulders-hunched and pressing on her thighs, perhaps it would work better with the hands dangling, a bit more "natural".
Personally, I think catchlights in the eyes are pretty much essential in a studio type portrait. The eyes are the most important part of the face, and you need to inject some light into them to get a good connection with the model. The angle you've got her looking at makes that really hard and is a tad unflattering.
I'm not a fan of the combination of the outfit, lipstick, hair colour and background combined. You've got a dark background, dark trousers and shoes, dark hair combined in a way where if the skin is exposed correctly, the background/clothes are going to lose detail.
Hopefully you will find this helpful - you've already decided that you're not entirely happy, so I'm confident you'll take it the right way and have another go - she's a good looking model! If you get her in a more middle-toned outfit and get a couple of lights to balance things, then you'll be much improved.
Rob