When working on a new WordPress plugin or generally speaking, a new feature of a project that will integrate with the WordPress back-end, I’m of the mind the all of the elements should inherit the styles provided by WordPress.
In short, I’m not a fan of when other people build things for the application and use the set of controls they think “look good” or that deviate from the core set of elements and style.
This isn’t to say that there aren’t improvements that can’t be made within WordPress, but there are also reasons for the look, feel, and usability of things as they currently are. And over time, I believe that these things will evolve just as they’ve done this far.
But what do you do when you’re working on a feature and you’re unsure of how it should work from a user experience perspective and you don’t really have a guide from which to draw?