The linked thread is good, but the short answer:

    Box<Foo>

If Foo is a struct, this is a single pointer to the heap. If Foo is a trait, this is a double pointer: a pointer to the data on the heap, and a pointer to a vtable for its methods. These two things are very different, but look similar. That's the mistake.

This change separates the two. Now:

    Box<dyn Foo>

is always the latter, a "trait object". We have to keep the old syntax working for stability reasons, but can lint against it, so that it nudges you into the better syntax. So in this world,

  // always a single pointer to a struct (or enum)
  Box<Foo>

  // always a double pointer
  Box<dyn Foo>

This is more clear for humans. The compiler can tell with 100% fidelity, of course. But humans aren't compilers. Different things with different costs look different.

As an addendum, the parent mentions another feature, "impl Trait", which can be used with traits. Their point was, originally, Box<Trait> was the only thing that existed like this. With "impl Trait", it was "impl Trait" vs "Trait", and is now "impl Trait" vs "dyn Trait", which is much more clear, at least in many people's opinions. :)