DB
Basically, Warrants are allowed to be specialists in their particular field, whereas commissioned officers are expected to grow in breadth (both mentally and physically
), e.g. Warrants that fly helos can expect to do so for their entire career, where a commissioned officer might get two tours under their belt before they are doomed to do staff jobs and the like.
I think this thread typifies why the USAF got rid of the Warrant Officer program years ago...
As for saluting within the US military structure, the enlisted salute the Warrants, and the Warrants salute the commissioned officer corps. As for other nations, from my experience it has been a matter of courtesy and not a requirement.
Cheers! M2