Joost de Valk has taken notice that there may be a large amount of plugins who’s setting pages in the newly redesigned WordPress back end will look awful. Joost is currently working on redoing all of his own plugins so they look good in the admin panel and has published a post which explains what he found in his research. The post goes over details such as the Wrapper, Headings, Individual Settings, and Submit Buttons.
For all of you plugin authors out there, be sure to check out the WordPress 2.5 Plugin Settings Pages Style Guide. If you are looking to migrate your theme or plugin to WordPress 2.5, check out this article on the WordPress Codex.