I don't think there is any one particular feature that is groundbreaking in Arc, although as a whole, Arc does have some interesting qualities. For example, code in Arc is much more succinct than code in other languages. When designing Arc, Graham would write a piece of code. He would then look at that piece of code and figure out what features would allow him to make that piece of code shorter. From there, he would implement that feature and rewrite the code. Due to this process, Arc has a large number of very simple utilities (e.g. check). With all of these utilities, code in Arc is much more concise than the equivalent code in other languages.
