Misinformation among anti-globalists
I think this feeling is exists among nut jobs on both sides of the traditional political divide. Unfortunately, it is based on a gross misunderstanding of the American legal system. The G20, and globalization generally, provides lots of good reasons for concern, but generic fear of international agreements overriding US sovereignty is not one of them.
For America, there is no such thing as international law
International treaties and laws don't become binding on US citizens automatically, or just because the president signed them. All international agreements must be approved by congress and signed by the president before the agreement becomes binding on Americans.
Congress cannot give up law making power to any other body. For example, Congress couldn't pass a law that says "from now on, the UN can make laws for the US." It would be unconstitutional. Laws cannot be constructed so as to bypass either/both houses because of the bicameralism principle. For example, congress passed a bill which gave power over deportation to the inspector general, but retained a veto over his actions for one house of congress. The issue came up to the Supreme Court, and a one house veto was considered unconstitutional because of bicameralism. Laws cannot be constructed to pass the president either, because of the presentment principle. US diplomats and negotiators have agreed to countless international laws, only to have the proposed laws killed or modified beyond recognition in congress.
All "international laws" must become national law before they are enforceable. I don't know about other countries, but this is how the governmental system in the US works.
Unconstitutional agreements, even if they became law, would not pass SCOTUS
The Supreme Court had the power to review any law or action binding on a US citizen. They've already held the principles of bicameralism, presentment, and non-delegation of lawmaking powers are unconstitutional. A law in violation of one of these principles wouldn't even get to SCOTUS, it would be killed by a district court.
For the specific fears of the person I was talking with, SCOTUS has already held handgun bans unconstitutional. Even if somehow a gun ban got past congress & the president, the Supreme Court would strike it down immediately. Ditto for free speech, the Supreme Court has long pushed free speech jurisprudence beyond what was popular at the time. It was the Supreme Court striking down libel suits, media shutdowns, and restrictive speech laws. The Supreme Court has demonstrated ability and willingness to strike down unconstitutional laws & actions
In sum, the president can't act unilaterally, the Supreme Court can strike down any law or government action abridging constitutional rights, and this broad fear of international agreement or cooperation is irrational and unfounded.
The response to my argument was, "well I guess we have nothing to fear, like when the IMF sacked Argentina." I don't know enough to evaluate the truth of that statement, but it doesn't matter. My statement was about US citizens fearing a loss of US sovereignty to international cooperation, not about Argentina being rescued by the IMF.
Again, there are lots of good reasons to worry about the G20.
You may worry the G20 doesn't have the capacity to make good decisions. Perhaps because the increased size (they used to be G8) will make it impossible to get anything substantive done. Or maybe they don't have the right framework for making decisions, or they're not willing to look at big underlying problems, they don't have the right long term focus, they don't have the power to get anything done, or any of a dozen other concerns.
You may worry they don't have your best interests in mind, but rather the interests of big business or environmentalists or the rich or the poor or some other group that doesn't include you. You may worry the G20 decisions will harm others, such as the excluded small developing nations, weaker included nations, or the poor in their own countries, or any other underrepresented group.
Like I said, the intellectually honest have plenty of reasons to worry about G20 decisions and deliberations, but the fear of an international body forcing laws on the US outside the democratic process should not be among them.