It's about abuse of power.
Don't get me wrong; race may very well have played a role in Crowley's decision to arrest Gates. Race certainly played a role in Gate's reaction to Crowley. The color of Gates' skin may have made the decision to arrest easier. However, issues of racial bias are deep and complex, and not easy to discern. It is clear that the police acted stupidly for two reasons.
First, being belligerent, obnoxious, rude, or impolite in your own home is not a crime. It is rarely a crime under any circumstances, but it is a clear abuse of power to arrest someone in their home for what amounts to being angry or upset. Abuse of arrest powers under these circumstances is far too common and, unfortunately, far too accepted. Many people even are so lacking in Constitutional sensibilities that they defend police when they make arrests for 'offense of cop.'
We should be clear about this. Arresting someone is very serious. Depriving someone of liberty is an act that should never be capricious or unwarranted. The founders of this country placed liberty right behind life itself when they enumerated the basic rights given by God to everyone. If you arrest someone for rude behavior, you are one step from finding it acceptable to kill someone for the same reasons.
Secondly, the arrest was stupid because of who Gates is. A well-known professor and historian. A friend of the President. It was completely predictable that this would become a public relations debacle. The reason, I am sure, that the prosecutors wasted no time in dropping the charges. As some have said, this situation was the 'perfect storm' of circumstances. Even without the perfect storm, arrests of this type will receive more and more scrutiny, and will more frequently result in bad press and damaged careers. There are simply too many records being made in the form of video and audio recordings, and the dispersal of these records is quick and easy. The police are being scrutinized like never before. Who would deny the stupidity of abusing your power while the world is watching? Sarah Palin might have a few words for you.
Yes, race was likely a factor in the arrest of Gates, but for who and how much is something that will be debated endlessly. The real problem with the arrest is that it was a clear abuse of police power, depriving a citizen of his liberty and removing him from his home when his only infraction was being upset in the presence of a police officer.