I am not a mom yet, but as a nurse, I have watched several circumcisions, both infant and adult.
As an infant, it really isn't all that distressing to them due to the local anesthetic that is applied.
As an adult, it's very painful and is basically considered major surgery where they go to the operation room and get placed under general anesthesia! Post-operatively is also very difficult and painful for several weeks as an adult, where as with a newborn, there isn't any noticed difficulties with pain.
Two of the biggest reasons for this procedure is religion (Jewish) and looks. What's not discussed, though is the health problems that can come if the boy isn't circumcised. If the boy is not taught good hygiene, then under the foreskin becomes a HUGE breeding ground for bacteria and yeast and in general, infections! There is also a risk of cutting off blood supply to the area if the foreskin doesn't get put back where it belongs and gets left pulled back, thus causing a tourniquet effect (castration).
DH also says that there can be an emotional impact to not being circumcised as well as an unconscious affiliation to poverty and not being circumcised when the boy gets old enough to start noticing the difference between him and his friends. DH was in the military for 17 years and as a healthcare provider, actually noticed that this was one of the #1 procedures the young men in bootcamp, basic training, and on board the ships wanted to go through even though it was so painful.
HTH