A thing I always didn't like of Bootstrap is that it requires you to load a full version of jQuery just to use it. Maybe I don't need the whole jQuery, because I use another framework, or because I have a very simple site that doesn't require any JavaScript at all.

The thing is that when Bootstrap first launched everyone was using jQuery, so it was a de facto standard, everyone already had it, so requiring it as a dependency was not an issue at all, now we are moving towards different web technologies and practically nobody is using jQuery, and having to load it just a dependency to Bootstrap is not good, considering that it will slow down the loading of the page and consume traffic for mobile users.