I'll try an answer from my limited experience.
The only JC hearing I sat in (as a fourth elder, the newbie) concerned a brother who left his wife and three kids and moved in with a younger woman. He agreed to attend the JC after the PO had contacted him at his work place. When questioned by the PO he confirmed the facts, stated he had no intention of reconciling with his wife and plainly said to all, "you've go to do what you've got to do."
It lasted 5 minutes and he was disfellowshipped (I didn't sign the forms but I agreed with the decision.)
Regret?, sadness? or a sense of justice? I was a real believer at that time but beginning to get my eyes opened. I was deeply sad that it came to DF'ing. I was sad that a man could walk out on his family like that. Was there justice? No. What was achieved? The elders didn't help.
Did you have doubts as to your qualifications to be making this decision?
I had lots of doubts but the decision was almost forced. The lack of qualifications was really in helping someone going through the pain of a separation. I was at a loss at what to say to the man's wife. I didn't believe that DF'ing someone was going to "shock" them to their senses.
Were you nervous, was there any doubt in your mind at all that this was the right decision?
At the time I didn't doubt it was right decision.
When it was over did you feel like God's Spirit had helped in the decision making?
No. The experience left me with more doubts about the whole process. The "brother" in this case had been through the JC process on more than one occasion previously and I felt some elders were glad to see the back of him. I had known the man since I was a kid but had only recently been back in the same congregation as him. It was difficult to face him in the JC setting.
Now, I regret ever being an elder. I didn't last long. After three years I disagreed with the elder body over another judical matter (lack of action) and resigned. I am glad I didn't get involved in any other JCs it could have been worse.
A couple of years later my wife left me to find someone else. The sort of mid-life crisis she went through reminded me of the DF'ing case I was involved with. I had stopped attending meetings by this time and I certainly was not going to blab to the elders about her.
She received some counseling from a psychologist. It didn't help our marriage but it helped her. At least she got some professional help, the very thing elders couldn't provide.
Thirdson