In the end the story of Stargate is Daniel's story, and has been right back to the film; as was shown by the dismal failure of most of Season 6 in his absence.
He is a genius, probably one of the most brilliant minds in the world but he has a profound lack of personal arrogance. He is happy as long as the solution is arrived at, he doesn't care about getting the credit. He can be tactless, but not from any sense of malice - he just assumes that everyone has his dedication to truth, even at the expense of personal ego. He has a snarky sense of humour, and he doesn't hold grudges often, but when he does, watch out.
His love for Sha're, long after any realistic hope of getting her back was admirable (and his initial resentment at her pregnancy was very human); his desire to help and protect her son showed the kind of man he really is.
His passion for doing the right thing may sometimes annoy those around him, but people of goodwill soon realise that he is often right, that the compromises and grey areas they have been accepting can and should be fought.
His courage, both physical and emotional, is profound; he has dealt with a succession of personal tragedies that would have destroyed a lot of people, and without becoming bitter or emotionally cold.
He has grown to think of his team and the SGC as family, and he would die to protect them, but then he would do and has done the same for a planet of strangers.
His friendship with Jack is one of the most important relationships in his life. They complement each other both personally and professionally. Daniel has someone he can rely on to back him up regardless of the situation, the good guys can only win if they stay alive. And Jack knows he has someone who enjoys negotiating so much more than he does, who can convince people that Earth has more to offer than "shoot first and ask questions later".
Posted at GW. Names have been ommitted to protect the ridiculous.