I love KDE4 and its look and feel, and am constantly amazed at how it works.
But the one thing that made my jaw drop was a relatively simple feature I discovered while using Konqueror the other day.
When using Firefox, you can highlight some text and do a search on it via the right-click menu. You can do this in Konqueror as well. But highlight an unlinked url text, and the right-click menu offers to open that url for you! This is so simple yet so amazing! I often find unlinked urls in forum posts and in mailing list archives so this feature is quite handy.
I sure hope this isn’t something that’s been around for a while, like Digikam/Kipi’s ability to upload photos to a Gallery album. That revelation almost made me fall out of my chair.