What does he mean by "regular objects"? (2:50)

21.3.8 Plain objects are not iterable 
Plain objects (as created by object literals) are not iterable:

for (const x of {}) { // TypeError
    console.log(x);
}
Why aren’t objects iterable over properties, by default? The reasoning is as follows. There are two levels at which you can iterate in JavaScript:

The program level: iterating over properties means examining the structure of the program.
The data level: iterating over a data structure means examining the data managed by the program.
Making iteration over properties the default would mean mixing those levels, which would have two disadvantages:

You can’t iterate over the properties of data structures.
Once you iterate over the properties of an object, turning that object into a data structure would break your code.
If engines were to implement iterability via a method Object.prototype[Symbol.iterator]() then there would be an additional caveat: Objects created via Object.create(null) wouldn’t be iterable, because Object.prototype is not in their prototype chain.

It is important to remember that iterating over the properties of an object is mainly interesting if you use objects as Maps1. But we only do that in ES5 because we have no better alternative. In ECMAScript 6, we have the built-in data structure Map.