The answer: not much. Pew Research has an article about the small number of issues in which 90% of Americans agree:
Yet there are some opinions that 90% of the public, or close to it, shares — including a belief that citizens have a duty to vote, an admiration for those who get rich through hard work, a strong sense of patriotism and a belief that society should give everyone an equal opportunity to succeed. Pew Research’s political values surveys have shown that these attitudes have remained remarkably consistent over time.
The proportion saying they are very patriotic has varied by just four percentage points (between 87% to 91%) across 13 surveys conducted over 22 years. Similarly, in May 1987, 90% agreed with the statement: “Our society should do what is necessary to make sure everyone has an equal opportunity to succeed.” This percentage has remained at about 90% ever since (87% in the most recent political values survey).
Interestingly, these cited figures are about foundational values in American culture. Exactly what some of these things mean could be up for debate: how should one express their “very patriotic” feelings? What exactly should it look like so that “everyone has an equal opportunity to succeed”? But as values, voting, patriotism, and meritocracy are quite powerful. (And it would also be interesting to see who doesn’t agree with these values.)
We could also ask why exactly 90% is a cutoff we should care about. Here is an explanation:
[R]eaching the 90% threshold is a rare occurrence in public opinion surveys. In part, this reflects the tendency of polling organizations to focus on current issues about which there are often considerable differences of opinion. Nonetheless, even on issues where one would expect to find near-total agreement, the public’s views are far from unanimous.
This is why Pew highlights a recent finding: “fully 90% of the public said that they were hearing mostly bad news about gas prices.”
It would be interesting to see more data on this to know just how rare 90% agreement is. How often might we expect to see this out of all survey responses? How different is the 90% occurrence compared to 80% or even 70%? Is this lack of 90% agreement unusual only for the United States or does this apply to other nations as well?