This ultimately is what shapes my view of what a good test is vs a bad test.

An issue I have with a lot of unit tests is they are too strongly coupled to the implementation. What that means is any change to the implementation ultimately means you have to change tests.

IMO, good tests are relatively immutable. You should be able to have multiple valid implementations. You should add new tests to describe the new functionality of that implementation, however, the old tests should remain relatively untouched.

If it turns out that a single change to an implementation requires you to change and update 20 tests, those are bad tests.

What I want as a dev is to immediately think "I must have broken something" when a test fails, not "I need to go fix 20 tests".

For example, let's say you have a method which sorts data.

A bad test will check "did you call this `swap` function 5 times". A good test will say "I gave the method this unsorted data set, is the data set sorted?". Heck, a good test can even say something like "was this large data set sorted in under x time". That's more tricky to do well, but still a better test than the "did you call swap the right number of times" or even worse "Did you invoke this sequence of swap calls".