It's impossible (and the things that would make it possible are pretty immoral) to arrest someone because of what they think, but if someone goes on Twitter, for example, and writes "I hate Obama because he's black" (that's a grotesque simplification, but you get the point), that person should, at least, be fined.
That would be quite interesting in a country with 300 million people. Probably have to hire thousands of new judges just for social media, not to mention specialized law enforcement officers to hunt down these criminals. What about forums? Life, if someone said that here, would the FBI subpoena VGC records and IP providers?