What would he do?

The President appoints the heads of all of our regulatory agencies, many of them can effectively create law. Obama let the head of the Education department force Universities to restructure how sexual investigations are handled resulting in presumption of guilt. Trump has let ICE run wild and fired up a trade war.

Those are very limited examples because they come from administrations that were/are sane and normal other than these issues, a real socialist would be setting their sights a lot higher.

A President Bernie could force any company which has a union trying to organize to let them organize. He could appoint people who want to restructure worker pay and impose problems on any company which does business with the government who has a CEO/worker pay ratio over a certain amount. He could let the EPA get very serious about believing Carbon Dioxide is a pollutant and effectively outlaw economic activity. He could have the FDA not approve new drugs unless they're "available for everyone". He could vastly expand the regulatory state by taking the most permissive view of what each agency can do and control.

Misusing the regulatory equipment which already exists in an effort to create a socialist utopia is the lower bound for what he can do.

The upper bound is he gets Congress to pass at least some of his agenda... and unfortunately that's the way to bet. I get that as a Venezuela style socialist he doesn't believe he's trying to trainwreck the economy, but that is the agenda.

On a side note Bernie claims Trump's trade war and immigration policies don't go anywhere close to far enough, that Trump is "soft" on these issues. I seriously doubt a President Bernie would be an improvement even there.

It's the agent problem. Can I really trust him to do X, especially when most people think it would destroy the republic?

A President Bernie would accomplish pretty much none of his crazy socialist schemes in the event he somehow won.

Just like Trump will never be able to build a wall. The President has a ton of power which doesn't depend on Congress and/or which has been delegated to him by Congress.

A President Bernie could mess with the economy on an impressive level, these abilities would not be well used. He wouldn't be able to impose socialism, but he could and would do a lot of damage. Further if "the people" are screaming for socialism to the point where Bernie's past is an advantage, then a Congressman's first duty is to be reelected.

I'd vote for Bloomberg. I haven't voted for Trump yet, but against Bernie I'll vote Trump and be happy about it.

It was almost like he was caught off guard by a question about the biggest topic of his tenure as Mayor. Yikes!

From his point of view it was a small issue. It's like a University President who increases enrollment, standards, endowments, makes multiple new buildings... and he's being asked about how the Football coach handled some player when everyone at the time thought it was a good idea.

It was a popular idea for a serious problem, it's now an unpopular idea.