As Stephan pointed out, the diamond is "out of round" as can be determined by looking at the spread in the "measurements" of the diamond as stated in the upper left region of the diamond grading report. The difference between 7.56 and 7.67 millimeters being more than 10/100th's however the diamond is not significantly out of round which is why it is not readily apparent to you now that it has been set, the odds are that the presence of the prongs even things out a bit... However for those people lurking about PS, a more significant difference, perhaps 7.56 - 7.70 would be pretty apparent. It is amazing what effect a few tenths of a millimeter can have upon the appearance of a round stone....
Actually our first thought was "Wow - M color, that's pretty warm" but some people prefer diamonds with a warmer tone... Todd's mother for instance... A concept which he finds rather horrifying since she's always telling people "Oh yes, I got this from my son" and he's thinking "Great! Now everybody thinks that I sell dingy yellow diamonds" but the fact is that there are diamonds for all sorts of people and lots of people like diamonds with a little warmth to them... So it is quite possible that you will be able to find a buyer for your diamond, maybe even here on Price Scope. However we suggest that you obtain an OGI or Sarin proportions analysis on the diamond so that (1) we can tell you what we think of the cut based upon the proportions, and (2) so that the people considering your stone may do so with all of the pertinent information at their fingertips.
The second thing that we noticed is the table diameter is pretty spread, but that may not be such an issue if the crown and pavilion angle are right... Just because a diamond is not "ideal cut" does not mean that it won't be full of life, it very well can be if the crown angle and more importantly the pavilion angle are in the right range...
You indicated that you just got the GIA report for your diamond... Are you the person who submitted the stone to the GIA for grading? If so, they should release the measurements for the crown and pavilion angle to you... They will not release that information to you however if you are not the person who submitted the diamond for grading because they consider it to be proprietory in nature.
