A conformation critique doesn't really make sense unless you have the context of what you want to do with her. There are lots of conformation issues I can totally live with on a horse who is already doing the job I want them to do that I wouldn't be psyched about in an upper level prospect.
That being said, cute mare, and if you and her owner like her and she's doing what you want her to do, then that should be enough, no?
The main thing I notice is that she's standing under herself in front. That can indicate heel pain, so you might want to get a second opinion on her hooves.
Her neck is placed nicely, though it could use a lot more muscle on top. It's a decent length, with a nice clean throatlatch. It could stand to be a hair longer, but I personally don't mind a shorter neck. I think the more muscle would make it look more proportional to the rest of her.
Her shoulder is quite straight, but at least it's nice and open and of good length. It might keep her from having flashy movement in the front end, but it shouldn't cause any problems.
Her back is a bit short, but better short than long. She does have good withers, which helps in keeping the saddle placed correctly. More muscle along her back will help her carry herself better.
Her hind end is fairly straight also, so she might have trouble really pushing off to jump very high. Her build also makes me think she might tend to leave her hind legs out behind instead of really reaching up underneath, but that may not be true. My mare doesn't look like she should be able to collect, but she really can.
She is also built downhill (stifle is higher than elbow). You can teach them to carry themselves more uphill, but she will probably move with a level balance.
So, get her hooves checked out to make sure her heels are not sore, and then do a lot of correct work, really driving the hind legs under her, so she can build muscle on top.
She's a cute mare with a nice relaxed expression, so she should be a lot of fun!
IMHO Hampton Bay just gave a thorough and spot on analysis.
Despite her "flaws", she presents an nice overall impression and looks like she is sweet and relaxed.
For future reference: try to take conformation pictures without boots or bell boots, with the horse looking straight ahead, front legs in line with each other, one back leg slightly behind the other, and not leaning or reaching forward, etc (which can make them appear to stand under themselves when they are not really built that way).
If you are interested in learning about conformation, there is a great column in the Practical Horseman every issue where horses are analyzed and ranked based on discipline, etc. I find that I learn lots reading those articles. The author (whose name I cannot recall) may also have a book that you could pick up, but as Got Spots mentioned, conformation can be discipline-specific (or certain attributes are more essential than others).
Best of luck with your lease!
"We can judge the heart of a man by his treatment of animals" Immanuel Kant
Also, the subtle camped forward stance of her hind feet, combined with the angle of the hairline and (from what I can see ) a slightly convex dorsal hoof wall on the hind feet indicates a very probable negative plantar angle of the coffin bones of the hind feet, which can directly lead to back pain and the bunched up overall stance. I would be looking at the hooves of this horse before evaluating anything else.
Her entire stance may change once the feet are changed.