There are many positive and negative impressions of Othello.
In Act I, scene i, even before we meet Othello, we have impressions of his character. Check out some of the comments made by Iago and Roderigo regarding his character and the derogatory comments made by the two.
Brabantio also has negative commentary about Othello, but the Duke of Venus speaks to Othello in a respectful manner. See Act I scene where the Duke calls Othello to court and he also needs to defend his actions regarding Desdemona.
Cassio also speaks to Othello in a respectful manner throughout, even after he is stripped of his lieutenantship.
All of the commentary shapes Othello's character. Eventhough he is considered brave, valiant and trustworthy, he seems to always have to live up to his reputation. He cannot take it for granted and maybe this is why he is so easily swayed by Iago. He does not have the self-confidence to question Iago's information.