Eric Mann wanted to be able to click the WordPress toolbar and scroll to the top of the site, like he could on Google+. He describes the process of submitting a patch to core, the various revisions, and then seeing it in use on his own sites and on WordPress.com.
He also has good advice for those contributing code for the first time:
Not every patch you submit to Trac will make it in to WordPress core. Some changes affect too narrow an audience to belong in the core codebase. If someone claims “that’s plugin territory” it’s because they think the number of users will benefit is just too small to warrant rolling your change in with the core project. Don’t take this personally.