Great composition, good choice of subject and I like the white background. The lighting is also rather good and accentuates the colour of the flower well.
I think you could have tipped over to a 6 if the background was a little whiter. The top right quadrant is a little grey and I think this probably cost you a point or two. Also, the flower seems a little bit soft to me. I have that lens too and find at the longer focal lengths it is really quite soft.