There's nothing wrong with the basic idea of a code of conduct, in and of itself. There are a lot of things wrong with the current crop:

1. They tend to be heavily politicized, pushing one group's societal views on the entire community.

2. Many attempt to police people's behavior on their own time outside the community.

3. They remain remarkably silent about certain kinds of uncivil behavior, such as public shaming campaigns or importing of outside political disagreements. Often the codes are written with input from people who frequently indulge in this behavior.

4. People with objections are often steamrollered and accused of various social sins for not going along, or a code is simply imposed on the community and all discussion is ignored or closed.

If you want these codes to be established without the angry opposition usually seen, you need to address these points.