In The Giver, what do you feel is the most important aspect of the Ceremony of Twelve? Also, in a Utopian society, what rules should the Community...

The Ceremony of Twelve is important because it is the one time in the community when differences are mentioned. 

Everything in Jonas’s community is about Sameness.  Everyone follows strict rules of behavior and has all of their choices made for them.  Individuality is discouraged.  The one time it is important is in determining what role citizens will play in the community.  For this, the community needs to take advantage of any individual traits that might have survived Sameness.

Everything in the community is designed to keep people from feeling uncomfortable.  Actually, the community is designed to prevent people from feeling at all.  This is what the dream telling and feeling telling rituals are about.  Any time someone has a feeling, the objective is to get rid of it right away.

The Ceremony of Twelve follows ten other ceremonies that promote conformity.  The only other one that doesn’t is the Ceremony of One.  Every child has a number from birth.  At the Ceremony of One they are also assigned a name.  It is one small part of individuality.

The Chief Elder explains why the Ceremony of Twelve is different.

Then the Chief Elder moved ahead in her speech. "This is the time," she began, looking directly at them, "when we acknowledge differences. You Elevens have spent all your years till now learning to fit in, to standardize your behavior, to curb any impulse that might set you apart from the group. (Ch. 7)

She notes that the ceremony honors their differences because they determine their futures.  The committee of elders carefully reviews these differences to assign jobs based on the traits that individuals possess.  It is the one time when it is okay to stand out from the group.

During the ceremony, the other community members do not really do more than sit and listen.  Everyone is expected to come to the ceremony, except the Receiver.  He comes on the day that Jonas is selected, to see Jonas’s part of the ceremony.  There are rituals that the citizens take part in during the ceremonies.  For example, the Ceremony of Loss and the Ceremony of Replacement happen then.  When a four year old dies, the family gets a replacement child during this ceremony.

Now, at this special Naming, the community performed the brief Murmur-of-Replacement Ceremony, repeating the name for the first time since the loss: softly and slowly at first, then faster and with greater volume, as the couple stood on the stage with the newchild sleeping in the mother's arms. (Ch. 6)

This takes place during the ceremony day.  The replacement child gets a name, and all of the other one year olds do too.

The Ceremony of Twelve is one of the main things that makes Jonas's community unique.  It is part of the dystopia, because it means that the community controls everyone's lives down to the last detail.  People can't choose their fates.  They are not even allowed to choose their professions.