In my opinion the composition is too centered and the focus is too soft. The colours also seem a little flat and the image size would be better off making use of the full 640 pixels.
You have missed on opportunity of maximise the potential of this shot with it being so small. It would have had much more impact if it had been 640 on one side. I like the DOF used. I think that the size will cause people to mark it down though. :(
Nice composition, however since the picture is so small I am unable to see enough detail to tell if the focus is sharp or not.
"File Size: No larger than 150 KB (153,600 bytes). Dimension: No dimension may be shorter than 160 pixels or longer than 640 pixels." You should try to have your challenge entries as close to the maximum allowed; Your dimensions are 20,139 bytes and 450 x 298 pixels. If you have received low scores it is probably due more to the size of the picture then to the content.
please read the tutorial for preparing images for challenges. you have no doubt lost a lot of votes due to your image size. as best i can tell, this would have scored a LOT higher if the image had been larger. don't be discouraged, and DON'T QUIT! better luck next time.
A bit on the small side - take full advantage of the 640 pixels on one side allowance. I like the way the flower is lit but would like to see it better focused.
Orchids are beautiful flowers and do make great subjects. I like that you took this outside, is it even a wild orchid? I think your focus is what lets you down most with this shot. I would like to b e able to see every detail in the petals and I can't. Overall, the colors could pop a little more, too, but that is easily fixable in post-processing. A larger image (submit as large as you can on DPC, it often helps) would also be better.