I feel that manners have one of the biggest affects on giving off a first impression to someone. As a person who works in retail, I get approached by people a lot for the first time in my life and I have but seconds of communication to get an initial impression from people and I would believe they are quite accurate. Any person who is rude or interruptive as their first encounter with someone is very telling, as is someone who is polite or respectful of a person. Over time you learn to gather cues from people, whether it be eye contact or body gestures to anticipate what a person may be like.
Manners and politeness mean everything when first talking to someone so don't forget that! Don't be that one person everyone is telling stories about being an insufferable person :)
However it is the case that in some households or families, the concept of certain manners protocols may not have been as apparent and after reaching a certain maturity they may reach a point where it is very hard to build a habit of the polite/respectful cues.