Someone suggested on the newbie thread to post here and get some constructive criticism. So here I am! 

Remember, I'm a newbie... so please be kind!  ... but don't be afraid to be honest.

You need to upload the full size if you want a real critique.  It looks like it might have some noise under the plate and be a bit over-compressed/over-sharpened and have artifacts throughout.  Honestly though that might just be because of the settings you used to save the thumb and might not be on the original.

every month IS sends out a newsletter with lots of useful articles.
if you know someone who is with IS, i suggest you ask them to send you some of these past newsletters.
another way to see if your work  is up to  IS standard may be to visit IS and look at the images there. if food is what you shoot, then look under that category.
i think food is a difficult category to shoot well, if you're new to this type of photography. i don't know your past photography experience , but i suggest going to IS and look over their images there.
then compare them to what you shoot best. and then go from there.

once again, if you are new to photography, i would stay away from isolated shots, food, and the more ambitious stuff. for now at least. but if you know how to light an object well, and get a white background ,etc.. then go for it.

i suggest try shooting in natural light first. there are less factors to complicate things for you.
one thing for sure, work to get the correct exposure before shooting.
pay close attention to shadow details and highlight , and make sure these are not blocked or clipped.  use fill in flash or reflectors to achieve this.
and then when all is right on , take the photo.

IS favours work that has little or no post processing, other than spotting and colour correction. unlike SS that tends to like the opposite.
be sure your focus is spot on too. don't use post processing to sharpen a poorly focused image. reshoot.

i hope that helps. and for the rest, as i said, visit IS site and look well.
 

I am just a newbie. But I'd suggest more exposure to get the light-weight look that all the stock pictures have in common. When you look at it it must pop out and feel good immediately. Turn up exposure a bit and don't stay on the cold side of colors, and turn up vibrance a bit (don't exagerate...). something like this: https://secure.istockphoto.com/stock-photo-10784480-stuffed-pork-tenderloin-roast-with-asparagus.php (even if this is just a bad example).
i can't tell much for the shadows and stuff because I am on an uncalibrated monitor at the moment.
Also I think the others are right, the plate is better white or some uniform color, maybe even a colorful plate like yellow could make it work but i dont know.
I find that the background things are a bit disturbing, but that's maybe just my taste.
Then you can also think about variations that make the picture interesting, such as a few cut slices with some red meat shown, or show a hand in the picture with the knife while cutting, a fork that takes one slice, someone taking it with heat-resistant gloves out of the oven, a hand adding a small piece of parsil as decoration on top or at a side as a final touch, and so on and on and on. These small details sometimes make the picture dynamic and interesting and not just plain and one-dimensional. The more dynamic pictures can tell a story and relate to a concept such in this case cooking, gourmet-stuff, but possibly also living at home, family, etc. When you take a picture, think of who might buy it, and in which ways it might be used, and you will find the story it must tell. :-)
you can type roastbeef into istock and see what others have done for example.
well i am just a beginner so don't consider too much what i am saying.
Good luck, I like your picture. :-)
Just have fun, try it, don't look at money, just enjoy and learn from this experience.

Simone

EDIT:
I forgot to say that most of these pictures are made with good lightning, such as strobe lights or off-camera speedlights. In the link i sent here you can see that there is a light source from the left, and one from the right/front and maybe others. These lights make a lot of difference in the pictures. If you have an off-camera flash you can do a lot of things with that. If not you can still use as first tries some lights you have around in the house (but don't mix warm and cool lights...). I am not a lighting expert (not at all!!), it was to tell you what makes a big part of the difference for those great popping pictures we find in magazines and websites. Experimenting with light is for me one of the most exciting aspects of photography.

http://www.istockphoto.com/stock-photo-6021948-tea-for-two.php

just spontaneous seeking within istock food section, i found this selection impressive enough for you to grasp what a good photograph can be.
notice the lighting is light without losing detail to lens flare , and the shadows if any are light . images like this is possible so long as you pay careful attention to your exposure.
if you're inexperienced in "seeing" lighting ratio,etc... and taking the best exposure, i would suggest taking a reading off a grey card then bracket +-1/2 , 1 stop,
and see what you have on your calibrated monitor.

most food shots are more or less arranged lighting, and thus, the photographer
have already done a lot of homework re lighting and exposure.
and most would be shooting on manual setting , well, at least i do.

but i wouldn't get too deep into the semantics of it all.
i would rather suggest you spend the next few days merely leafing through these
awesome section and just sponge off as much info you can into your "mind's eye"
as to how these photographers light their food shot,
and how they expose them.

once it sinks in, then you can pick up the camera and experiment.
for now, i suggest a lot more looking than shooting.

iStock is good for rejecting images for something minor then when you resubmit, they reject it for something else. I personally wish they would tell you everything they see rather than just the one thing and save you the trouble of resubmitting and resubmitting. Most of the time they make it through. For instance I had submitted a photo which had a person in it and they rejected it because I forgot the model release. I understood...my fault...resubmitted with model release and then it was rejected for something else. So they are good for that

Remember, I'm a newbie... so please be kind!  ... but don't be afraid to be honest.

Is this an image the would be BEST to demonstrate your technical abilities? Would you submit it to a photo contest? Would this be part of a portfolio you would present to a customer? Would you apply for a photography job with this image?

I think those questions are what needs to be asked when you are applying to any agency. It's not about "just any image", it's about the best you can deliver. The same as with any application to a job: You present your BEST skills in the interviews, everybody knowing that your day-by-day routine will not always require those skills.