Anyone who has every copied and pasted from Word into WordPress (or any browser textbox for that matter), is familiar with the... interesting way the resulting text looks. WordPress, as of version 3.9, deals with this for you automatically — along with several other handy improvements.
Marko Heijnen of Six Revisions has put together a concise list — animations included — showing off the major improvements in the blog platform's latest version. 3.9 comes with a raft of media-related features, but it's the smaller tweaks — such as the auto-cleansing of Word text — that make it worth the upgrade.
I tend to use the HTML editor rather than the visual one in my WordPress installs, but if you (or your site users) are attached to WYSIWYG, this functionality alone is worth its weight in bitcoins.
3.9 also introduces live previewing of widgets, so you can see how they'll look on your site before the changes are saved.
Exciting Improvements in WordPress 3.9 [Six Revisions]