I'm guessing that these are the main reasons why people don't like him from what I've seen with boxing fans.

1. There is a perceived image of arrogance to him.
2. He got a lot of press due to his olympic glory and making very good money as he turned pro and going to PPV so early in his career.

Now to address these 2 points from my point of view. Most boxers have a degree of arrogance to him, yes even the so called "humble" ones like Pac. Just see the prefight talking and almost every boxer has a degree of arrogance. The 2nd point a lot of people might be a little pissed that he's making so much money and getting so much exposure where they felt he hasn't earned it. But ask yourself, if you were in Khan's shoes would you turn down the money and exposure? So I think those 2 points are the biggest issues that boxing fans have with Khan.

And 1 last thing, quite a few people are saying that Khan is cherry picking opponents (like Otley), I thought so at first too but have changed my mind since the Maidana fight. You don't cherry pick guys like Maidana and Bradley in a span of 7 months. One guy is the hardest puncher of the division and 1 guy is the no. 1 in the division. Those are 2 very good opponents. That's just irrational hating to say Khan is cherry picking opponents at this stage of his career.