The next Attorney General of the United States will be in charge of enforcing all federal laws, including Nixon's Controlled Substances Act, which lists cannabis as a schedule 1 hallucinogen.
Holder has followed Obama's lead; he has been permissive of medical and recreational shops selling buds in the open. But will the next guy be? What we need is an act of Congress (or the Court), amending the anachronistic controlling legislation. Until then-
The next guy may be a Mormon, or even worse, a Republican. It's a good job Holder's resigning now, to give his boss time to hire and indoctrinate his replacement.
Regardless, one thing the new AG can't do is conscript local or state law enforcement officers to enforce his objectives. And the crux of any law is the enforcement of it. Short of expanding the DEA until it's larger than the military (or deputizing soldiers to make arrests), city governments will dictate policy - under the command and control of the people.