Ordinarily, 1-2 ppl should be vertical, horizontal is more head and shoulders - which you nailed :thumbup:.
The image is off-centered - some ppl like it, others will come there with a ruler. If shooting and selling your work, keep that in mind.
HAIR - oh the lovely hair in the eyes. Again, if it is your partner in life or sister or whom ever and you are just playing, then
you are just playing but when selling this, she isn't a pirate thus seeing both eyes would very nice.
Lighting: Sunny day or not, lighting is probably THE MOST important aspect of photography. Under her eyes are shadows (raccoon eyes) you need a fill-flash here, should have used the built-in flash, it would also create catch light which to me are of very high importance during portraiture. The shadows around her are nice but again the ones under her eyes
do not glamor her.
For Futurebe careful when shooting on a sunny day: #1 ppl will squint and that doesn't look good, sun throws harsh shadows, thus either find a shade, or use a reflector to shade the subject or even use reflector to
direct the light where you want it to go. BRIGHT DAY doesn't mean you can use your flash - at least to fill in the shadows just a drop and to create catch lights.
Shooting at wide apertures: you shot this at ISO 100 (ok it is a sunny day) 1/1000 (ok, it's a sunny day) at 2.8 -
WHY? If you're focusing on her eye (b/c you can't focus on both eyes) then one of them will be crispy sharp and second won't be (depending on how far you're from her) and in this case, it seems like your focus is actually on the right side of the face (hair covering the eye) thus her nose and other parts of the face that are not in same plane of focus are not focused on. Hence the image seems soft and out of focus.
BE VERY CAREFUL when shooting at wide apertures, even at 4.5 you can mess up based on reasons above. For that reason and that reason only, normal portraits I shoot anywhere b/n 5.6-11 (lots of factors at play). ALSO most lenses are sharpest at around 1-2 stops down from their widest - in your case it is probably 5.6 anyways. Situations that I'd shoot wide open portraits - for example I have the bride bring the flowers to her face, she puts the hand with engagement ring on her face flowers, I focus on the ring and throw everything else out of focus, things like that.
now that you know how to alter your technique keep on shooting and improving!!!
good luck
P.S.
On this thread
http://www.thephotoforum.com/forum/...-three-useful-youtube-series-photography.html someone posted nice videos, look over them

good luck