It appears to me that people are more offended when they are on the receiving end of it. It also seems to be less offensive if you make a racist comment about a group you are a part of. I did laugh when at least two of my friends separately said they refused to pay gold to expand the stash in Diablo 3 because "they were asian". And then I didn't either, and well... -_-;
In any case, unsolicited racist comments are definitely not appreciated. You have to know someone well enough to do that kind of stuff.
It's only natural for human beings to stereotype; there's too many things in the world to process, so we simplify it and look for patterns. But racism is basically the combination of many truths into a lie. While the original traits for a stereotype may have existed (abeit exaggerated) the flaw is in assuming this is a universal truth that all people of a certain group are like that just because some might be.
In the end, be considerate of others before you say or so something. Otherwise you come across as a huge douchebag and playing the "You shouldn't be so serious" just doubles your douche rating.