This is a lovely shot. Why, - because of the lovely wistful expression; the beautiful eyes; the hand under the chin. Its just lovely all round.
Now for the critique part, and some very useful tips you may not be aware of.
First, focusing with your camera. There are a number of focus points, - those red squares that flash when the focus beeps; you can, and should select just ONE of these to be the focus point, ideally the one that will be over the subjects near eye. This is easily selectable, - that button at the top right camera back with the squares on it that you always wondered about, - press it, and move the focus points with multicontroller. This will ensure the critical part of the shot is tack sharp.
Next is prime lenses. You will not get a sharper image, or better contrast than you will with a prime lens, AND it will force you to think about composition, because you have to move around rather than zoom. BUT this lens you have works best at f/5.6, - though the depth of field will be deeper. Give it a try for fun.
For a portrait shot like this, try SPOT metering rather than multi segment, - again, just to explore the camera, and also get the perfect exposure for the skin (This is quite good as it is)
ALWAYS shoot in RAW, - this will give the ability to correct exposure, white balance, and numerous other things before you commit the shot to posterity.
Use this workflow when preparing a shot to upload:
Open original shot which has been sharpened>re size by setting pixels per inch to 72, and then set the longest side to whatever is allowed by the site>save as new file name>during save, adjust file size to that allowed by the site>save>OPEN new file, check sharpness at 50%>add sharpening as needed. This is because file compression can cause loss of sharpness.
I have to ask, - what post processing did you perform with this shot?
It is a little over saturated, and tends a little too much towards RED. Could be you over saturated, could be white balance. Canon cameras tend to favour red by the way, - including the top of the line 1 series.
So, being such a beautiful image, Ive loaded 4 mods.
Mod1 is you original with some colour balance and de saturation; with a small amount of sharpening; mod2, is a cloer crop, no other adjustments; mod3 is mod1 with a diffuse glow filter (Using CS4) applied, just to make it dreamlike; and mod4, - my favourite, is a mono conversion. I used the colour balance tool to reduce red and add more yellow.
I hope all of this is helpful, and that you like the mods.
One last thing, - if you are using Internet Explorer as your browser, - dont. Use Firefox or Safari 5 as they will display web colour properly, Explorer will not.