You can customize pretty much everything in Drupal, except a lot of administration UX. For example, did you ever just want your clients to have a button at the news view to simply add a news node using the overlay module? You can do that (and a lot of other cool stuff like automatic node reference) using the contextual administration module which depends on ctools (but you are already using ctools aren't you?).
Used correctly this module can improve your sites' usability. Read Kris' blog post about it and check some of the great video's avalaible on the project page to see some examples.