Beginning in version 8, Drupal does not load jQuery by default. Great - no issues for us! However, the long, shadowy spectre of jQuery haunted development.
Even if you’re not using jQuery, it can still load for anonymous users.
You have your libraries.yml file free of any dependencies, and you’re deep into development when you notice jQuery is still loading. Why? Contrib modules can still require jQuery.
For us, the culprit was the Google Analytics module. It’s a great contributed module, but because of the overhead it brought, we decided to create a custom module and attach our tracking code separately.
More to come.
I’m sure there will be other gotchas as D8 proliferates in the wild, but hopefully these two will save you some time in case you find yourself wondering, much like I was, why jQuery still cast its shadow. Now get back to it, and build something awesome.