Somebody told me once that I was judgmental. She was wrong. I am opinionated, yes. But not judgmental, definitely not.
My definition is; an opinion is to be refined, polished, changed if necessary. If you come to a new interpretation, new understanding of a person, or a situation then you adjust your opinion. An opinion is your own idea based on your own perception.
You are judgmental if you measure people (mostly) around you according to common beliefs, dogmatic reasoning, conform guidelines. Being judgmental means in my opinion (haha) that you are not thinking independently.