Plugins tend to very seldom render forms in a conventional, easily styled manner. There are rarely filters and/or plugin settings sufficient enough for getting the forms to look and contain exactly how and what you want them to.
But more importantly: plugin forms don't share a conventional markup, which leads to tons of CSS to make them fit your design.
I suggest a form field API that provides functions to render form fields of all kinds. The API would work with a set of keys and options, send all fields through filters (which many plugins don't) and wrap them properly in the markup.
For instance, something like
get_form_field( 'form_id', 'email', $attributesOrValue ); would produce consistent, conventional and customizable markup for all of us.
And of course, it would save developers much hazzle of rendering forms using loops, mixing PHP and HTML.