In several places core and plugins make use of the browser built-in functions confirm, alert and prompt.

These dialogs can't be styled. All they can do is show a message, an OK and a Cancel button. Chrome displays such dialogs usually at the top of the page while firefox centers them on the screen.
And Firefox allows you to suppress the built-in dialogs.

So it would be nice if jenkins provides a way to make use of the dialog tag that all modern browsers support, that allow to show a modal dialog. We can have our own buttons, freely choose the text of the buttons, color them for dangerous actions.

We can even put complete forms in such dialogs, e.g. when we need to enter 2 values, or we want to add something without the need to submit the form and have a page reload.

Another kind of anti-pattern is using a dedicated page for simple actions, e.g. to disconnect an agent on has to click the corresponding link and a new page loads where one can enter the reason and confirm. This could be also achieved with a modal dialog that pops up.

There are probably many more places where one could think of using a dialog instead of a dedicated page.