looks almost like two different horses. If you cover up her neck/head, and look only at the body, it looks like many AQHA's look. Tack on the neck, and suddenly the body looks under-boned in relation to the long neck and large head.
I think her pasterns are fine, length and angel. Her hock is good, and hind end looks fine. Seeing the tail head is part of the way her's comes out, though I agree that she is underweight and undermuscled.
Her neck is very long and straigh, and tied on to the shoulder low. The lack of bend (I mean like how a Swan bends it's neck, not side to side) in her neck will make it harder for her to lift it up, from the base , and lighten her shoulders. Same old stuff you heard and know.
You can ride her up hills at walk , over cavaletti or logs, and do lots of transitions. But you cannot changer her neck totally. She looks like a very sweet horse and though not perfect, is certainly no throw away .