The new version, now available as a beta from Objective Development's web site, adds a load of new features, including a built-in Clipboard history (that survives restarts), the ability to add items to the existing Clipboard rather than overwriting the contents, support for Quick Look, and integration with iCal for creating events and to-do items.
Veteran users like myself will delight in knowing that LaunchBar 5 also has a completely redesigned preferences window, replacing the old (and severely lacking) version. You can also choose to exclude LaunchBar from the Dock, which is really the best way to use it, but previously you had to exclude it from the dock via or by editing plist files. Now it's done by the check of a box and the press of a button. LaunchBar's also now scriptable via AppleScript, comes with a bunch of new automatic actions (such as compressing files and controlling apps) and improved support for writing your own custom actions via AppleScript or shell script.
There's a whole lot more in there, which I look forward to discovering as I play with the new beta. Of course, we'll have a full review when the product finally goes final. In the interim, that explains why I love, love, love LaunchBar so very much.