There were many interesting and beautiful contributions from the Fall 2005 Math21a class. I had started in December to scan through the submissions and worked many hours in January on that too. Chosing was hard and therefore, the list of graphics became longer than reasonable. Here are some criteria:

Originality. I took the first author of something which looked new. Later copies of the same object would be discarded. I'm aware that the original might so have become renamed. But I have no way to find out, who got inspired by intuition and who got inspired by somebody else.
Implementability. Since I had to retype thousands of Mathematica lines by hand, a submission of an individual student had to be short enough. Some nice but long graphics descriptions had to be dismissed because of that.
Potential. Some graphics had the potential to look good, when animated. I'm sure that I missed some nice graphics contributions but I had to choose, during the maybe 50 hours, I could spend on regenerating and enhancing the graphics.
Presentation. The way, the notebook was submitted also played a role. Of course, color coudl help to make a graphics stand out. I would also look more carefully at submissions, where the author had taken care to present it nicely.
Luck I worked during many days on this gallery. Sometimes for several hours in a row. At the end of a day, I would probably be less inclined to include something, then after a fresh start. But since the decision whether a graphics would be included in this gallery or not has no influence on the grades, this can be accepted.
Surprise. Some contributions were very simple, but had an element of surprise to me. If somebody can produce with a handful of graphics objects something meaningful, I would be tempted too to include it.