Jest has this wonderful feature where it suggests other equality checks you might want to consider if your test fails. It's super helpful and a really nice touch.
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Assertions are a surprisingly nuanced design space. In a test, if I assert `x < y`, I really want to see the values of x and y when it fails.
Do you define an API for every possible predicate (Python's assertLess, expect.js) or try to support the native syntax (c.f. pytest)?
I love how the CommonMark Spec has a test suite that's just a JSON array. It's really easy to test a library for compliance, and I've seen developers nerd-sniped into full compliance.
https://spec.commonmark.org/0.31.2/spec.json
I'm still experimenting with UIs for live (sandboxed) evaluation of tests. I've realised that you really want to highlight the failing assertion, not just the failing test.
Feedback welcome :)