It's a pleasure NorthStone, i love critiquing.

- Horizon lines: I'm guessing due to interference. Is this because they're trying to take the eye's focus away from the subject? Looking at the picture this seems to be what happens.
It just looks cleaner and is less of a distraction. The same goes with trees and branches that look like they are coming out of a model's head when posed in front of a tree. Try to avoid it if possible. In this case raising or lowering your camera position would have solved this.

- Background: I'm guessing too much white in the clouds has overexposed the sky. Thanks for the flash tip. It's not something I'd considered during an outdoor day shot. Is this because it will accentuate skin tones? Are these filters expensive?
No, it's because the camera cannot capture all of those tones in one exposure. The exposure range is too great. Therefore we underexpose the background a bit so that the sky is not blown. Then we add fill flash because if we did not the model's face would be too dark. IN GENERAL, flash will only light the foreground. Doing this helps balance an unmanageable 1 frame exposure. In terms of the filters...not that expensive, depends on brand. (35.- 200.) I'd leave the filters for now though (except a polarizing filter) and concentrate on getting familiar with flash and ambient exposures.

I think the model (my girlfriend that is) enjoyed that particular pose.
She does look relaxed and if she's happy then you're probably happy
But as a viewer with no stake in the shot, she's not giving the viewer much interaction.

best!
Marko